[Uploadable] add a null check before calling is_file to suppress deprecation warning

Author: JoolsMcFly

src/Uploadable/UploadableListener.php

@@ -389,7 +389,7 @@ public function processFile(AdapterInterface $ea, $object, $action)
      */
     public function removeFile($filePath)
     {
-        if (is_file($filePath)) {
+        if (!is_null($filePath) && is_file($filePath)) {
             return @unlink($filePath);
         }