Only count on arrays or countables to avoid warnings in PHP 7.2

Author: wouterj

src/Symfony/Component/Form/Form.php

@@ -748,7 +748,7 @@ public function isEmpty()
 
         return FormUtil::isEmpty($this->modelData) ||
             // arrays, countables
-            0 === count($this->modelData) ||
+            ((is_array($this->modelData) || $this->modelData instanceof \Countable) && 0 === count($this->modelData)) ||
             // traversables that are not countable
             ($this->modelData instanceof \Traversable && 0 === iterator_count($this->modelData));
     }