[Dialog] Don't forward hover event when there is nowhere to forward it to

Dialog has this clever way of re-sending a mouse event inside the bounardies of the dialog,
so despite visual padding, the items can be clicked on any corner following Fitt's law.
However, when the mainItem has no size, adjusting the position will adjust it inside the border
since there is no pixel inside the main item, leading to infinite recursion as the event is
processed again and again and again leading to a crash.

Differential Revision: https://phabricator.kde.org/D20200