debugindexstats: handle the lack of Rust support better
We don't have any stats in the Rust index. Currently it is not known which
stats would be interesting to get, so if they end up being important, we can
add them later.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}