API: Merge Request Accept fails with 405 in case no pipeline is given and `merge_when_pipeline_succeeds` is set to `true`

The current behavior of the Merge Request Accept API is that it yields a 405 METHOD NOT ALLOWED error in case the merge_when_pipeline_succeeds field is true, but the pipeline doesn't exist. I kinda understand why that may not be a valid request, however, I think it makes clients considerably easier to implement if the field is gracefully ignored in case the MR branch doesn't run a pipeline. IMHO, the current implementation isn't a measure of "protecting" against accidental merges, because that's what the project configured Pipeline must succeed flag is for.

Related: #353984