revset: make "null" able to appear in internal _list() expression

This is the same workaround introduced at e16456831516. Without this patch,
"null or x" can't be optimized to _list(null x).

Test case will be added by the next patch.

mercurial/revset.py

@@ -1928,7 +1928,8 b' def _list(repo, subset, x):'
         r = repo[t].rev()
         if r in seen:
             continue
-        if r in subset:
+        if (r in subset
+            or r == node.nullrev and isinstance(subset, fullreposet)):
             ls.append(r)
         seen.add(r)
     return baseset(ls)