Accessibility: Do not assert when a widget doesn't have a QAI

[ChangeLog][QtGui] Fixed crash when sending accessibility updates when
the corresponding widget does not have a corresponding
QAccessibleInterface. This showed on Mac for example with QStatusBar.

Task-number: QTBUG-35421
Change-Id: I94174e98e858b7a0122532ee5fcc8458a263bccd
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>