[Security] Return null instead of empty username to fix deprecation notice

Author: phasdev

Authenticator/RemoteUserAuthenticator.php

@@ -45,6 +45,6 @@ protected function extractUsername(Request $request): ?string
             throw new BadCredentialsException(sprintf('User key was not found: "%s".', $this->userKey));
         }
 
-        return $request->server->get($this->userKey);
+        return $request->server->get($this->userKey) ?: null;
     }
 }

Tests/Authenticator/RemoteUserAuthenticatorTest.php

@@ -36,6 +36,7 @@ public function testSupportNoUser()
         $authenticator = new RemoteUserAuthenticator(new InMemoryUserProvider(), new TokenStorage(), 'main');
 
         $this->assertFalse($authenticator->supports($this->createRequest([])));
+        $this->assertFalse($authenticator->supports($this->createRequest(['REMOTE_USER' => ''])));
     }
 
     public function testSupportTokenStorageWithToken()