Changing default value of isDirty to empty array so hasChanges works properly

See laravel/framework#20130 for what was
changed in Laravel 5.5. We now need to use an empty array instead
of null since the method hasChanges() in Illuminate\Database\Eloquent\Concerns\HasAttributes
is checking for an empty array instead of null.

Author: EricTendian

src/Hookable.php

@@ -139,10 +139,10 @@ public function save(array $options = [])
     /**
      * Register hook for isDirty.
      *
-     * @param null $attributes
+     * @param array $attributes
      * @return bool
      */
-    public function isDirty($attributes = null)
+    public function isDirty($attributes = [])
     {
         if (! is_array($attributes) && !is_null($attributes)) {
             $attributes = func_get_args();