formatter: fix handling of None value in templater mapping

For historical reasons, None in mapping dict means there's no such keyword,
and falls back to b"". That's fine in log templates where mapping item is
generally a callable returning a value (which may be None,) but the formatter
directly puts an "evaluated" value in the mapping. So the None value has
to be lifted to wrappedvalue(None) to avoid confusion in the template engine.

mercurial/formatter.py

@@ -515,6 +515,10 b' class templateformatter(baseformatter):'
         if part not in self._parts:
             return
         ref = self._parts[part]
+        # None can't be put in the mapping dict since it means <unset>
+        for k, v in item.items():
+            if v is None:
+                item[k] = templateutil.wrappedvalue(v)
         self._out.write(self._t.render(ref, item))
 
     @util.propertycache

tests/test-config.t

@@ -122,7 +122,7 b' Test config default of various types:'
   ]
   $ hg config --config auth.cookiefile= auth -T'json(defaultvalue)'
   [
-   {"defaultvalue": ""}
+   {"defaultvalue": null}
   ]
   $ hg config --config auth.cookiefile= auth -T'{defaultvalue}\n'