ci: Only run the sanity job if there's a MR

ci: Only run the sanity job if there's a MR

As it will fail right away if there isn't, and that prevents the CI to
run on people's branches.

$ ci-fairy check-merge-request --require-allow-collaboration --junit-xml=check-merge-request.xml
ERROR: No open merge request against mesa/mesa with sha 9f6aba4be0ceb59d35732651cc44f676e93b70b5

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Fixes: d4151f2e ("ci: Run sanity job only in pre-merge pipelines")
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8032>