retrigger_mr_pipeline_job: prevent rare exception where MR has no pipeline
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.