[8.x] Fix bug where columns would be guarded while filling Eloquent m…

…odels during unit tests (#39880)

* Fix bug where columns would be guarded while filling Eloquent models during unit tests

* Update GuardsAttributes.php

Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Database/Eloquent/Concerns/GuardsAttributes.php

@@ -217,9 +217,14 @@ public function isGuarded($key)
     protected function isGuardableColumn($key)
     {
         if (! isset(static::$guardableColumns[get_class($this)])) {
-            static::$guardableColumns[get_class($this)] = $this->getConnection()
+            $columns = $this->getConnection()
                         ->getSchemaBuilder()
                         ->getColumnListing($this->getTable());
+
+            if (empty($columns)) {
+                return true;
+            }
+            static::$guardableColumns[get_class($this)] = $columns;
         }
 
         return in_array($key, static::$guardableColumns[get_class($this)]);