ignore spurious resize events to empty sizes

Summary:
on multiscreen systems, the notification dialog gets resized to 0x0
right after componentcomplete by someone else (either kwin, notification
applet or Qt code) causing 382340, resizing a window to a null size is not correct anyways
and should never happen.
with this, all other resize events are correct and notifications are ok on multiscreen
systems

BUG:382340

Test Plan:
system with primary monitor and panel in the external monitor
at the right of a panel, notifications are fine

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D7127