Change return type in _getParentHref

It never returns null because for null it is overwritten with empty string

Classes/Common/MetsDocument.php

@@ -1215,7 +1215,7 @@ protected function _getToplevelId()
  /**
  * Try to determine URL of parent document.
  *
- * @return string|null
+ * @return string
  */
  public function _getParentHref()
  {