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

mercurial/obsutil.py

@@ -782,7 +782,7 b' def successorsandmarkers(repo, ctx):'
     # closestsuccessors returns an empty list for pruned revisions, remap it
     # into a list containing an empty list for future processing
     if ssets == []:
-        ssets = [[]]
+        ssets = [_succs()]
 
     # Try to recover pruned markers
     succsmap = repo.obsstore.successors