persistent-nodemap: avoid writing nodemap for empty revlog

The format cannot encode the lack of tip_rev.

There is currently nothing known to write such empty nodemap right now, but the
change we are preparing on default reveal this issue. So I had rather fix it on
stable.

mercurial/revlogutils/nodemap.py

@@ -163,6 +163,8 b' def delete_nodemap(tr, repo, revlog):'
 
 def persist_nodemap(tr, revlog, pending=False, force=False):
     """Write nodemap data on disk for a given revlog"""
+    if len(revlog.index) <= 0:
+        return
     if getattr(revlog, 'filteredrevs', ()):
         raise error.ProgrammingError(
             "cannot persist nodemap of a filtered changelog"