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

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

```release-note
NONE
```

model/command_webhook.go

@@ -53,11 +53,11 @@ func (o *CommandWebhook) IsValid() *AppError {
 		return NewAppError("CommandWebhook.IsValid", "model.command_hook.channel_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if len(o.RootId) != 0 && !IsValidId(o.RootId) {
+	if o.RootId != "" && !IsValidId(o.RootId) {
 		return NewAppError("CommandWebhook.IsValid", "model.command_hook.root_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if len(o.ParentId) != 0 && !IsValidId(o.ParentId) {
+	if o.ParentId != "" && !IsValidId(o.ParentId) {
 		return NewAppError("CommandWebhook.IsValid", "model.command_hook.parent_id.app_error", nil, "", http.StatusBadRequest)
 	}
 

model/config.go

@@ -3484,13 +3484,13 @@ func (s *ServiceSettings) isValid() *AppError {
 		return NewAppError("Config.IsValid", "model.config.is_valid.login_attempts.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if len(*s.SiteURL) != 0 {
+	if *s.SiteURL != "" {
 		if _, err := url.ParseRequestURI(*s.SiteURL); err != nil {
 			return NewAppError("Config.IsValid", "model.config.is_valid.site_url.app_error", nil, "", http.StatusBadRequest)
 		}
 	}
 
-	if len(*s.WebsocketURL) != 0 {
+	if *s.WebsocketURL != "" {
 		if _, err := url.ParseRequestURI(*s.WebsocketURL); err != nil {
 			return NewAppError("Config.IsValid", "model.config.is_valid.websocket_url.app_error", nil, "", http.StatusBadRequest)
 		}

model/file_info.go

@@ -114,7 +114,7 @@ func (fi *FileInfo) IsValid() *AppError {
 		return NewAppError("FileInfo.IsValid", "model.file_info.is_valid.user_id.app_error", nil, "id="+fi.Id, http.StatusBadRequest)
 	}
 
-	if len(fi.PostId) != 0 && !IsValidId(fi.PostId) {
+	if fi.PostId != "" && !IsValidId(fi.PostId) {
 		return NewAppError("FileInfo.IsValid", "model.file_info.is_valid.post_id.app_error", nil, "id="+fi.Id, http.StatusBadRequest)
 	}
 

model/outgoing_webhook.go

@@ -140,7 +140,7 @@ func (o *OutgoingWebhook) IsValid() *AppError {
 		return NewAppError("OutgoingWebhook.IsValid", "model.outgoing_hook.is_valid.user_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if len(o.ChannelId) != 0 && !IsValidId(o.ChannelId) {
+	if o.ChannelId != "" && !IsValidId(o.ChannelId) {
 		return NewAppError("OutgoingWebhook.IsValid", "model.outgoing_hook.is_valid.channel_id.app_error", nil, "", http.StatusBadRequest)
 	}
 

model/scheme.go

@@ -159,15 +159,15 @@ func (scheme *Scheme) IsValidForCreate() bool {
 	}
 
 	if scheme.Scope == SCHEME_SCOPE_CHANNEL {
-		if len(scheme.DefaultTeamAdminRole) != 0 {
+		if scheme.DefaultTeamAdminRole != "" {
 			return false
 		}
 
-		if len(scheme.DefaultTeamUserRole) != 0 {
+		if scheme.DefaultTeamUserRole != "" {
 			return false
 		}
 
-		if len(scheme.DefaultTeamGuestRole) != 0 {
+		if scheme.DefaultTeamGuestRole != "" {
 			return false
 		}
 	}

model/search_params.go

@@ -214,7 +214,7 @@ func parseSearchFlags(input []string) ([]searchWord, []flag) {
 			// and remove extra pound #s
 			word = hashtagStart.ReplaceAllString(word, "#")
 
-			if len(word) != 0 {
+			if word != "" {
 				words = append(words, searchWord{
 					word,
 					exclude,
@@ -345,9 +345,9 @@ func ParseSearchParams(text string, timeZoneOffset int) []*SearchParams {
 		len(excludedPlainTerms) == 0 && len(excludedHashtagTerms) == 0 &&
 		(len(inChannels) != 0 || len(fromUsers) != 0 ||
 			len(excludedChannels) != 0 || len(excludedUsers) != 0 ||
-			len(afterDate) != 0 || len(excludedAfterDate) != 0 ||
-			len(beforeDate) != 0 || len(excludedBeforeDate) != 0 ||
-			len(onDate) != 0 || len(excludedDate) != 0) {
+			afterDate != "" || excludedAfterDate != "" ||
+			beforeDate != "" || excludedBeforeDate != "" ||
+			onDate != "" || excludedDate != "") {
 		paramsList = append(paramsList, &SearchParams{
 			Terms:              "",
 			ExcludedTerms:      "",