Resolve #3485352 "Throwing error in"

Closes #3485352

src/FileMetadata.php

@@ -183,7 +183,7 @@ class FileMetadata implements FileMetadataInterface {
    */
   public function getMetadata($metadata_id, $key = NULL) {
     try {
-      if ($plugin = $this->getFileMetadataPlugin($metadata_id)) {
+      if ($plugin = $this->getFileMetadataPlugin($metadata_id) && !empty($key)) {
         $metadata = $plugin->getMetadata($key);
       }
       else {