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 9f6aba4b
Signed-off-by: Tomeu Vizoso tomeu.vizoso@collabora.com