Clarify that we zero out the widget coords and only keep its dimensions.

If we have no widget to fall-back to, memset to 0 the output @rect since
we return FALSE whether or not we have widget, so protect users from not
knowing if there was a widget and possibly accessing uninitialised ints.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/893#note_1766079