fix: only validate data from POST request body

With the current data validation check, an empty POST request with valid GET parameters will cause the validation to pass but credentials which are later fetched from POST request body will be null.

Author: Miguel Renaud-Nolte

src/Controllers/LoginController.php

@@ -47,7 +47,7 @@ public function loginAction(): RedirectResponse
         // like the password, can only be validated properly here.
         $rules = $this->getValidationRules();
 
-        if (! $this->validate($rules)) {
+        if (! $this->validateData($this->request->getPost(), $rules)) {
             return redirect()->back()->withInput()->with('errors', $this->validator->getErrors());
         }