Fix UserLoginType::getBlockPrefix() return type for Symfony 8.1 compatibility

AbstractType::getBlockPrefix() return type changed from ?string to string in
Symfony 8.1, making the ?string override a fatal incompatible declaration.

Author: silverbackdan

src/Form/Type/User/UserLoginType.php

@@ -77,7 +77,7 @@ public function buildForm(FormBuilderInterface $builder, array $options): void
             );
     }
 
-    public function getBlockPrefix(): ?string
+    public function getBlockPrefix(): string
     {
         return '';
     }