Do not attach null buffer.

Apparently Weston does not like this and causes instant crashes when
hiding a top-level widget.  The QtCompositor-side behavior is dubious
too although at least that does not crash. Nevertheless attaching a
null buffer seems unnecessary.

Change-Id: I1d7fe3e0b05e5d77773b661a71bc37311968eded
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>