fix preg_match() parameter #2 must be string

preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in /vendor/laravel/framework/src/Illuminate/Routing/UrlGenerator.php on line 625

src/Auth/Database/Administrator.php

@@ -47,7 +47,7 @@ public function __construct(array $attributes = [])
      */
     public function getAvatarAttribute($avatar)
     {
-        if (url()->isValidUrl($avatar)) {
+        if ($avatar && url()->isValidUrl($avatar)) {
             return $avatar;
         }