[Qt] Crash in gmail on enabling desktop notifications
https://bugs.webkit.org/show_bug.cgi?id=106699

Patch by David Rosca.
Reviewed by Jocelyn Turcotte.

Protect against callback that may be null. This also matches WebKit2 behaviour.

* WebCoreSupport/NotificationPresenterClientQt.cpp:
(WebCore::NotificationPresenterClientQt::allowNotificationForFrame):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140322 268f45cc-cd09-0410-ab3c-d52691b4dbfc