revset: prevent crash caused by empty group expression while optimizing "and"
An empty group expression "()" generates None in AST, so the optimizer have
to test it before destructuring a tuple. The error message, "missing argument",
is somewhat obscure, but it should be better than crash.
<?xml version="1.0" encoding="{encoding}"?>
<feed xmlns="http://www.w3.org/2005/Atom">