minor #19142 [Security] [Guard] Improve comment with working example (pasdeloup)

This PR was squashed before being merged into the 2.8 branch (closes #19142).

Discussion
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[Security] [Guard] Improve comment with working example

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Improve the code given in the comment to make it work.
If you follow blindly the previous example (as I did) and never return null, you're stucked in an infinite redirection loop on your login page.

Commits
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b36fc32 [Security] [Guard] Improve comment with working example

src/Symfony/Component/Security/Guard/GuardAuthenticatorInterface.php

@@ -39,11 +39,15 @@ interface GuardAuthenticatorInterface extends AuthenticationEntryPointInterface
      * Whatever value you return here will be passed to getUser() and checkCredentials()
      *
      * For example, for a form login, you might:
-     *
-     *      return array(
-     *          'username' => $request->request->get('_username'),
-     *          'password' => $request->request->get('_password'),
-     *      );
+     * 
+     *      if ($request->request->has('_username')) {
+     *          return array(
+     *              'username' => $request->request->get('_username'),
+     *              'password' => $request->request->get('_password'),
+     *          );
+     *      } else {
+     *          return;
+     *      }
      *
      * Or for an API token that's on a header, you might use:
      *