minor symfony#7912 pass only strings to loadUserByUsername() (xabbuh)

This PR was merged into the 2.8 branch.

Discussion
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pass only strings to loadUserByUsername()

This completes symfony#7065 as since the change made there the `token` key can refer to `null`.

Commits
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a47ed88 pass only strings to loadUserByUsername()

Author: xabbuh

security/guard_authentication.rst

@@ -186,6 +186,10 @@ This requires you to implement six methods::
         {
             $apiKey = $credentials['token'];
 
+            if (null === $apiKey) {
+                return;
+            }
+
             // if null, authentication will fail
             // if a User object, checkCredentials() is called
             return $userProvider->loadUserByUsername($apiKey);