window-x11: Fix windows that set empty input shapes

Windows that set empty input shapes get n_rects of 0 when querying them
later, which makes sense, but the code that interpreted the result
translated it into a NULL input shape, which meant it was the same as
the bounding region. As such, an empty input shape would actually get
interpreted as a full input shape!

We, ourselves, set an empty input shape on tray icon windows in
gnome-shell since we would handle the picking ourselves. This meant that
we'd actually get the MetaSurfaceActorX11 when hovering over the tray
icon, instead of the ShellGTKEmbed that we capture events on and react
to.

This fixes weird tray icon behavior in gnome-shell.