[ticket/12572] Fix JS error when alert message title is undefined

JS console reports error in core.js:
TypeError: alert is undefined
alert.hide()
when res.MESSAGE_TITLE is undefined.

PHPBB3-12572
Merge pull request #2480 from rxu/ticket/12572

[ticket/12572] Fix JS error when alert message title is undefined

* rxu/ticket/12572:
  [ticket/12572] Fix JS error when alert message title is undefined