UserMergeBranch does not verify it got a user name and email
While UserMergeBranch verifies that it got a user as part of the first frequest, it doesn't verify that the user actually has a name and email set. This leads to an Internal
error at a later point when trying to create the merge commit because you cannot create merge commits with either of them being empty. We should improve our validation to check these parameters and return an InvalidArgument
error if unset.