exchange: allow passing no includes/excludes to `pull()`
I would expect that `exchange.pull(includepats=[])` results in an empty list of
include patterns to be passed to the remote, but it doesn't currently because we
check for any truthy value instead of checking specifically for `not None`.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}