Fix pipelines for merge requests in project exports

Previously, an import for a pipeline for merge request would fail because the project.json did not export the merge_request hash in the tree. The pipeline would fail to validate with, "Validation failed: Merge request can't be blank".

Unlike !17799 (merged), this change won't make existing exports importable, but it will properly establish a relationship between a pipeline and a merge request for future exports.

Relates to #31414 (closed)