templater: fix string representation of wrapped None

flatten() and stringify() skip None, which means wrappedvalue(None).show()
must return '' instead of 'None'. This isn't a problem right now, but we'll
encounter it once we start using wrapped types extensively.

mercurial/templateutil.py

@@ -155,6 +155,8 b' class wrappedvalue(wrapped):'
         raise error.ParseError(_('%r is not iterable') % self._value)
 
     def show(self, context, mapping):
+        if self._value is None:
+            return b''
         return pycompat.bytestr(self._value)
 
     def tovalue(self, context, mapping):