Merge branch '6.4' into 7.2

* 6.4:
  [WebProfilerBundle] Fix tests
  [Cache] Tests for Redis Replication with cache
  [BrowserKit] Fix submitting forms with empty file fields
  [WebProfilerBundle] Fix interception for non conventional redirects
  [DependencyInjection] Do not preload functions
  [DependencyInjection] Fix cloned lazy services not sharing their dependencies when dumped with PhpDumper
  [HttpClient] Fix activity tracking leading to negative timeout errors
  [Security] Return null instead of empty username to fix deprecation notice
  [DependencyInjection] Fix env default processor with scalar node

Authenticator/RemoteUserAuthenticator.php

@@ -44,6 +44,6 @@ 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()