[9.x] Add hasUser method to Guard contract (#39167)

* [9.x] Add `hasUser` method to Guard contract

- #24518 in this PR was added method `hasUser` to the GuardHelper trait. As for me it will be useful to have this method in the contract.

* Update Guard.php

Co-authored-by: Tetiana Blindaruk <t.blindaruk@gmail.com>
Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Contracts/Auth/Guard.php

@@ -40,6 +40,13 @@ public function id();
      */
     public function validate(array $credentials = []);
 
+    /**
+     * Determine if the guard has a user instance.
+     *
+     * @return bool
+     */
+    public function hasUser();
+
     /**
      * Set the current user.
      *