libweston: Avoid emitting an output if none set
This is an odd corner case where the surface doesn't yet have an output assigned -- noticed when starting up with the RDP backend and fullscreen-shell, and we attempt to emit an timeline point without an output, causing the RDP backend to crash. Rather than trying to catch this in the timeline code, still emit the flush damage timeline but without any output.
Signed-off-by: Marius Vlad marius.vlad@collabora.com