[5.8] Added hasUser method to Guard contract;

 - laravel#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.

src/Illuminate/Contracts/Auth/Guard.php

@@ -47,4 +47,11 @@ public function validate(array $credentials = []);
      * @return void
      */
     public function setUser(Authenticatable $user);
+
+    /**
+     * Determine if the guard has a user instance.
+     *
+     * @return bool
+     */
+    public function hasUser();
 }