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

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

```release-note
NONE
```

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)
 		}