bug #40758 [Security] NullToken signature (jderusse)

This PR was merged into the 5.2 branch.

Discussion
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[Security] NullToken signature

| Q             | A
| ------------- | ---
| Branch?       | 5.2
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

The signature of `TokenInterface::getUser` does not allow returning null. But `NullToken` returns `null`.
https://github.com/symfony/symfony/blob/0f96ac74847d114c9d9679655bcf3e94b6ba69d1/src/Symfony/Component/Security/Core/Authentication/Token/TokenInterface.php#L49-L56

This PR update `NullToken::getUser` to return an empty string instead of null.
I wonder if the fix shouldn't be to change the return type in the interface, but that would be a BC break, right?

Commits
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9ad3720efc [security] NullToken signature

Authentication/Token/NullToken.php

@@ -33,7 +33,7 @@ public function getCredentials()
 
     public function getUser()
     {
-        return null;
+        return '';
     }
 
     public function setUser($user)