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

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

```release-note
NONE
```

api4/channel.go

@@ -1447,7 +1447,7 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	postRootId, ok := props["post_root_id"].(string)
-	if ok && len(postRootId) != 0 && !model.IsValidId(postRootId) {
+	if ok && postRootId != "" && !model.IsValidId(postRootId) {
 		c.SetInvalidParam("post_root_id")
 		return
 	}

api4/config_test.go

@@ -29,18 +29,18 @@ func TestGetConfig(t *testing.T) {
 
 		require.NotEqual(t, "", cfg.TeamSettings.SiteName)
 
-		if *cfg.LdapSettings.BindPassword != model.FAKE_SETTING && len(*cfg.LdapSettings.BindPassword) != 0 {
+		if *cfg.LdapSettings.BindPassword != model.FAKE_SETTING && *cfg.LdapSettings.BindPassword != "" {
 			require.FailNow(t, "did not sanitize properly")
 		}
 		require.Equal(t, model.FAKE_SETTING, *cfg.FileSettings.PublicLinkSalt, "did not sanitize properly")
 
-		if *cfg.FileSettings.AmazonS3SecretAccessKey != model.FAKE_SETTING && len(*cfg.FileSettings.AmazonS3SecretAccessKey) != 0 {
+		if *cfg.FileSettings.AmazonS3SecretAccessKey != model.FAKE_SETTING && *cfg.FileSettings.AmazonS3SecretAccessKey != "" {
 			require.FailNow(t, "did not sanitize properly")
 		}
-		if *cfg.EmailSettings.SMTPPassword != model.FAKE_SETTING && len(*cfg.EmailSettings.SMTPPassword) != 0 {
+		if *cfg.EmailSettings.SMTPPassword != model.FAKE_SETTING && *cfg.EmailSettings.SMTPPassword != "" {
 			require.FailNow(t, "did not sanitize properly")
 		}
-		if *cfg.GitLabSettings.Secret != model.FAKE_SETTING && len(*cfg.GitLabSettings.Secret) != 0 {
+		if *cfg.GitLabSettings.Secret != model.FAKE_SETTING && *cfg.GitLabSettings.Secret != "" {
 			require.FailNow(t, "did not sanitize properly")
 		}
 		require.Equal(t, model.FAKE_SETTING, *cfg.SqlSettings.DataSource, "did not sanitize properly")

api4/user.go

@@ -559,7 +559,7 @@ func getFilteredUsersStats(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	channelRoles := []string{}
-	if channelRolesString != "" && len(channelID) != 0 {
+	if channelRolesString != "" && channelID != "" {
 		channelRoles, rolesValid = model.CleanRoleNames(strings.Split(channelRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("channelRoles")
@@ -567,7 +567,7 @@ func getFilteredUsersStats(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	teamRoles := []string{}
-	if teamRolesString != "" && len(teamID) != 0 {
+	if teamRolesString != "" && teamID != "" {
 		teamRoles, rolesValid = model.CleanRoleNames(strings.Split(teamRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("teamRoles")
@@ -673,7 +673,7 @@ func getUsers(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	channelRoles := []string{}
-	if channelRolesString != "" && len(inChannelId) != 0 {
+	if channelRolesString != "" && inChannelId != "" {
 		channelRoles, rolesValid = model.CleanRoleNames(strings.Split(channelRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("channelRoles")
@@ -681,7 +681,7 @@ func getUsers(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	teamRoles := []string{}
-	if teamRolesString != "" && len(inTeamId) != 0 {
+	if teamRolesString != "" && inTeamId != "" {
 		teamRoles, rolesValid = model.CleanRoleNames(strings.Split(teamRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("teamRoles")