hgweb: fix type of {nav} keyword when linerange filter is active
Spotted by making '%' operator check the operand type more strictly. Before,
"{nav}" would be evaluated to a lazy "" (empty string) and {"" % x} would
yield one empty string.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}