hgtagsfnodescache: handle nullid lookup

The null revision is empty, so it `.hgtags` content is `nullid` in regards with
the `hgtagsfnodescache`. Dealing with `nullid` will help with the next
changeset. Before this change, feeding `nullid` to `hgtagsfnodescache.getfnode` would
return a wrong result (fnode for tip).

mercurial/tags.py

@@ -691,6 +691,9 b' class hgtagsfnodescache(object):'
         If an .hgtags does not exist at the specified revision, nullid is
         returned.
         """
+        if node == nullid:
+            return nullid
+
         ctx = self._repo[node]
         rev = ctx.rev()