There are rare cases where a merge request does not have a pipeline, even
though this might only occur under testing circumstances, e.g.:

https://gitlab.com/garz-fricke/yocto/infrastructure/ci-test/minimal-foo/-/merge_requests/58

The retrigger_mr_pipeline_job script throws an exception in these cases:

https://gitlab.com/garz-fricke/yocto/infrastructure/ci-test/minimal-manifest/-/jobs/1345385316

Fix this by checking for existing pipeline before querying it.