ProjectMergeRequest.commits() has surprising returns #266

I'm not sure why exactly, but it seems that .commits() (v4) has three kinds of returns:

• An actual list of ProjectCommit objects
• An empty list (even though the merge request has commits attached to it)
• Raising a 404 error

I'm finding this very puzzling, especially since the endpoint is not documented to return a 404.

(This is all on the same project, btw.)