context: fallback function should not return None

mercurial/context.py

@@ -685,7 +685,7 b' class workingctx(changectx):'
         def findflag(ctx):
             mnode = ctx.changeset()[0]
             node, flag = self._repo.manifest.find(mnode, orig)
-            ff = self._repo.dirstate.flagfunc(lambda x: flag or None)
+            ff = self._repo.dirstate.flagfunc(lambda x: flag or '')
             try:
                 return ff(path)
             except OSError: