shell: Don't start a fade animation if we don't have an output

shell: Don't start a fade animation if we don't have an output

When the last window of the X11 compositor is closed during a fade or
while locked, we'll try to start a fade back to the lock screen.  However,
if we closed the last window, there are no outputs left and the animation
will try to run with surface->output == NULL.

https://bugs.freedesktop.org/show_bug.cgi?id=73665