fix: accept only valid data from POST

With the current data validation check, an empty POST request with valid GET parameters will cause the validation to pass but a TypeError will be thrown because on line 110 it is fetching null data from POST.

Author: Miguel Renaud-Nolte

src/Controllers/RegisterController.php

@@ -100,7 +100,7 @@ public function registerAction(): 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());
         }