xdg: fix error when launching windowed Chromium

xdg: fix error when launching windowed Chromium

Chromium sends 2 commits before the commit with a buffer attached. We
were just checking `wlr_box_empty(&view->pending)` to handle the cases
where an initially maximized/fullscreen client is windowed, but that
check was also returning true on the 2nd commit from Chromium, resulting
in an error message: "view has empty geometry, not centering".