repoview: prevent `None` to be passed as the filtername
We let such instantiation slip in a previous commit, so we add an explicit check
to prevent it to happen in the future.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}