background-manager: Add don't emit a NULL bg in background-changed
background-manager: Add don't emit a NULL bg in background-changed

When the background's are finalised we ask the manager to provide another
to replace it if needed this used to mean we emitted a null background
when we were shutting down.

media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer