weston: Handle no pointer event

weston: Handle no pointer event

This should fix weston-stacking crashing if there's no pointer surface
due to not getting any pointer event. There's an implicit assumption
that we have pointer events if we also have keyboard input focus which
in AGL shell we don't have.

Bug-AGL: SPEC-4916
Change-Id: I2b043108bf41307779bf683f2685701653c1117f
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29263
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>