minor #10552 Supports() must return a boolean (darrylhein)

This PR was submitted for the 4.1 branch but it was merged into the 3.4 branch instead (closes #10552).

Discussion
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Supports() must return a boolean

The code docs say "null" when it should be false.

Commits
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4052f6a supports() must return a boolean

Author: javiereguiluz

security/guard_authentication.rst

@@ -590,7 +590,7 @@ are two possible fixes:
         public function supports(Request $request)
         {
     +         // if there is already an authenticated user (likely due to the session)
-    +         // then return null and skip authentication: there is no need.
+    +         // then return false and skip authentication: there is no need.
     +         if ($this->security->getUser()) {
     +             return false;
     +         }