fixed is_file(): Passing null to parameter filename of type string is…

… deprecated (#71)

src/UploadBehavior.php

@@ -282,7 +282,7 @@ protected function save($file, $path)
     protected function delete($attribute, $old = false)
     {
         $path = $this->getUploadPath($attribute, $old);
-        if (is_file($path)) {
+        if (is_string($path) && is_file($path)) {
             unlink($path);
         }
     }