branchcache: avoid created a `None` filter repoview when writing

The repoview class is not intended to be used for unfiltered repository.

mercurial/branchmap.py

@@ -168,7 +168,10 b' class BranchMapCache:'
         unfi = repo.unfiltered()
         for filtername, cache in self._per_filter.items():
             if cache._delayed:
-                repo = unfi.filtered(filtername)
+                if filtername is None:
+                    repo = unfi
+                else:
+                    repo = unfi.filtered(filtername)
                 cache.write(repo)