desktop: Fix destruction of applications that fallback to the desktop role

desktop: Fix destruction of applications that fallback to the desktop role

With commit 9827cfd25578, application not having a valid app_id will
fallback to being assigned the desktop role. In this case, destroying
those kind of surfaces will not have an output assigned, so add a
code path for those as well. Found while testing cluster-receiver with
waylandsink and sub-surfaces.

Bug-AGL: SPEC-3382

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: If20bdcf02810c16167fdbfbf30f3fd367c43069b