check if username is empty in usercontroller

src/Controller/UserController.php

@@ -290,7 +290,11 @@ class UserController implements IAuthenticationRequired
         }
 
         $newUsername = \Container::$request->post('username');
-        if (strlen($newUsername) > 0 && $newUsername !== $user->getUsername()) {
+        if ($newUsername !== $user->getUsername()) {
+            if (strlen($newUsername) == 0) {
+                return new JsonContent(['error' => ['errorText' => 'Username cannot be empty.']]);
+            }
+
             if (preg_match('/^[a-zA-Z0-9_\-\.]+$/', $newUsername) !== 1) {
                 return new JsonContent(['error' => ['errorText' => 'Username can contain only english letters, digits, - (hyphen), . (dot), _ (underscore).']]);
             }