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

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

```release-note
NONE
```

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")