usermergebranch: add validation for user.{email, name}

While 'UserMergeBranch' verifies that it got a user as part of the first request, 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.

This commit improves our validation to check these parameters and return an InvalidArgument error if unset. We add a new test 'TestUserMergeBranch_failure' to test the validation logic in 'UserMergeBranch'.

fixes: #4257 (closed)