obsutil: maintain a homogenous list when computing successors In practice, this wasn't a problem because of an internal check for an empty list, but it avoids the following pytype error: File "/mnt/c/Users/Matt/hg/mercurial/obsutil.py", line 823, in successorsandmarkers: No attribute 'markers' on List[nothing] [attribute-error] In Union[Any, List[nothing], _succs] Differential Revision: https://phab.mercurial-scm.org/D10213