MM-31062: Rewrite empty string checks to be more idiomatic (#16587)

https://mattermost.atlassian.net/browse/MM-31062

```release-note
NONE
```

app/channel.go

@@ -629,7 +629,7 @@ func (a *App) CreateChannelScheme(channel *model.Channel) (*model.Scheme, *model
 
 // DeleteChannelScheme deletes a channels scheme and sets its SchemeId to nil.
 func (a *App) DeleteChannelScheme(channel *model.Channel) (*model.Channel, *model.AppError) {
-	if channel.SchemeId != nil && len(*channel.SchemeId) != 0 {
+	if channel.SchemeId != nil && *channel.SchemeId != "" {
 		if _, err := a.DeleteScheme(*channel.SchemeId); err != nil {
 			return nil, err
 		}
@@ -784,7 +784,7 @@ func (a *App) GetSchemeRolesForChannel(channelId string) (guestRoleName, userRol
 		return
 	}
 
-	if channel.SchemeId != nil && len(*channel.SchemeId) != 0 {
+	if channel.SchemeId != nil && *channel.SchemeId != "" {
 		var scheme *model.Scheme
 		scheme, err = a.GetScheme(*channel.SchemeId)
 		if err != nil {
@@ -808,7 +808,7 @@ func (a *App) GetTeamSchemeChannelRoles(teamId string) (guestRoleName, userRoleN
 		return
 	}
 
-	if team.SchemeId != nil && len(*team.SchemeId) != 0 {
+	if team.SchemeId != nil && *team.SchemeId != "" {
 		var scheme *model.Scheme
 		scheme, err = a.GetScheme(*team.SchemeId)
 		if err != nil {

app/command.go

@@ -520,7 +520,7 @@ func (a *App) DoCommandRequest(cmd *model.Command, p url.Values) (*model.Command
 
 func (a *App) HandleCommandResponse(command *model.Command, args *model.CommandArgs, response *model.CommandResponse, builtIn bool) (*model.CommandResponse, *model.AppError) {
 	trigger := ""
-	if len(args.Command) != 0 {
+	if args.Command != "" {
 		parts := strings.Split(args.Command, " ")
 		trigger = parts[0][1:]
 		trigger = strings.ToLower(trigger)
@@ -561,7 +561,7 @@ func (a *App) HandleCommandResponsePost(command *model.Command, args *model.Comm
 	post.Type = response.Type
 	post.SetProps(response.Props)
 
-	if len(response.ChannelId) != 0 {
+	if response.ChannelId != "" {
 		_, err := a.GetChannelMember(response.ChannelId, args.UserId)
 		if err != nil {
 			err = model.NewAppError("HandleCommandResponsePost", "api.command.command_post.forbidden.app_error", nil, err.Error(), http.StatusForbidden)
@@ -573,20 +573,20 @@ func (a *App) HandleCommandResponsePost(command *model.Command, args *model.Comm
 	isBotPost := !builtIn
 
 	if *a.Config().ServiceSettings.EnablePostUsernameOverride {
-		if len(command.Username) != 0 {
+		if command.Username != "" {
 			post.AddProp("override_username", command.Username)
 			isBotPost = true
-		} else if len(response.Username) != 0 {
+		} else if response.Username != "" {
 			post.AddProp("override_username", response.Username)
 			isBotPost = true
 		}
 	}
 
 	if *a.Config().ServiceSettings.EnablePostIconOverride {
-		if len(command.IconURL) != 0 {
+		if command.IconURL != "" {
 			post.AddProp("override_icon_url", command.IconURL)
 			isBotPost = true
-		} else if len(response.IconURL) != 0 {
+		} else if response.IconURL != "" {
 			post.AddProp("override_icon_url", response.IconURL)
 			isBotPost = true
 		} else {

app/login.go

@@ -122,7 +122,7 @@ func (a *App) GetUserForLogin(id, loginId string) (*model.User, *model.AppError)
 	enableEmail := *a.Config().EmailSettings.EnableSignInWithEmail
 
 	// If we are given a userID then fail if we can't find a user with that ID
-	if len(id) != 0 {
+	if id != "" {
 		user, err := a.GetUser(id)
 		if err != nil {
 			if err.Id != MISSING_ACCOUNT_ERROR {

app/notification.go

@@ -780,10 +780,10 @@ func getMentionsEnabledFields(post *model.Post) model.StringArray {
 	ret = append(ret, post.Message)
 	for _, attachment := range post.Attachments() {
 
-		if len(attachment.Pretext) != 0 {
+		if attachment.Pretext != "" {
 			ret = append(ret, attachment.Pretext)
 		}
-		if len(attachment.Text) != 0 {
+		if attachment.Text != "" {
 			ret = append(ret, attachment.Text)
 		}
 	}

app/notification_email.go

@@ -314,7 +314,7 @@ func (a *App) generateHyperlinkForChannels(postMessage, teamName, teamURL string
 }
 
 func (s *Server) GetMessageForNotification(post *model.Post, translateFunc i18n.TranslateFunc) string {
-	if len(strings.TrimSpace(post.Message)) != 0 || len(post.FileIds) == 0 {
+	if strings.TrimSpace(post.Message) != "" || len(post.FileIds) == 0 {
 		return post.Message
 	}
 

app/oauth.go

@@ -415,11 +415,11 @@ func (a *App) newSessionUpdateToken(appName string, accessData *model.AccessData
 func (a *App) GetOAuthLoginEndpoint(w http.ResponseWriter, r *http.Request, service, teamId, action, redirectTo, loginHint string, isMobile bool) (string, *model.AppError) {
 	stateProps := map[string]string{}
 	stateProps["action"] = action
-	if len(teamId) != 0 {
+	if teamId != "" {
 		stateProps["team_id"] = teamId
 	}
 
-	if len(redirectTo) != 0 {
+	if redirectTo != "" {
 		stateProps["redirect_to"] = redirectTo
 	}
 
@@ -436,7 +436,7 @@ func (a *App) GetOAuthLoginEndpoint(w http.ResponseWriter, r *http.Request, serv
 func (a *App) GetOAuthSignupEndpoint(w http.ResponseWriter, r *http.Request, service, teamId string) (string, *model.AppError) {
 	stateProps := map[string]string{}
 	stateProps["action"] = model.OAUTH_ACTION_SIGNUP
-	if len(teamId) != 0 {
+	if teamId != "" {
 		stateProps["team_id"] = teamId
 	}
 

app/team.go

@@ -342,7 +342,7 @@ func (a *App) GetSchemeRolesForTeam(teamId string) (string, string, string, *mod
 		return "", "", "", err
 	}
 
-	if team.SchemeId != nil && len(*team.SchemeId) != 0 {
+	if team.SchemeId != nil && *team.SchemeId != "" {
 		scheme, err := a.GetScheme(*team.SchemeId)
 		if err != nil {
 			return "", "", "", err

app/user.go

@@ -1497,7 +1497,7 @@ func (a *App) SendPasswordReset(email string, siteURL string) (bool, *model.AppE
 		return false, nil
 	}
 
-	if user.AuthData != nil && len(*user.AuthData) != 0 {
+	if user.AuthData != nil && *user.AuthData != "" {
 		return false, model.NewAppError("SendPasswordReset", "api.user.send_password_reset.sso.app_error", nil, "userId="+user.Id, http.StatusBadRequest)
 	}
 

app/webhook.go

@@ -264,7 +264,7 @@ func (a *App) CreateWebhookPost(userId string, channel *model.Channel, text, ove
 	}
 
 	if *a.Config().ServiceSettings.EnablePostUsernameOverride {
-		if len(overrideUsername) != 0 {
+		if overrideUsername != "" {
 			post.AddProp("override_username", overrideUsername)
 		} else {
 			post.AddProp("override_username", model.DEFAULT_WEBHOOK_USERNAME)
@@ -272,10 +272,10 @@ func (a *App) CreateWebhookPost(userId string, channel *model.Channel, text, ove
 	}
 
 	if *a.Config().ServiceSettings.EnablePostIconOverride {
-		if len(overrideIconUrl) != 0 {
+		if overrideIconUrl != "" {
 			post.AddProp("override_icon_url", overrideIconUrl)
 		}
-		if len(overrideIconEmoji) != 0 {
+		if overrideIconEmoji != "" {
 			post.AddProp("override_icon_emoji", overrideIconEmoji)
 		}
 	}
@@ -445,7 +445,7 @@ func (a *App) CreateOutgoingWebhook(hook *model.OutgoingWebhook) (*model.Outgoin
 		return nil, model.NewAppError("CreateOutgoingWebhook", "api.outgoing_webhook.disabled.app_error", nil, "", http.StatusNotImplemented)
 	}
 
-	if len(hook.ChannelId) != 0 {
+	if hook.ChannelId != "" {
 		channel, errCh := a.Srv().Store.Channel().Get(hook.ChannelId, true)
 		if errCh != nil {
 			var nfErr *store.ErrNotFound
@@ -696,7 +696,7 @@ func (a *App) HandleIncomingWebhook(hookId string, req *model.IncomingWebhookReq
 	var channel *model.Channel
 	var cchan chan store.StoreResult
 
-	if len(channelName) != 0 {
+	if channelName != "" {
 		if channelName[0] == '@' {
 			result, nErr := a.Srv().Store.User().GetByUsername(channelName[1:])
 			if nErr != nil {