Consistent handling of empty data in vtiles

[springmeyer]

trafficstars

Rendering zero features to a vtile using map.render(vtile,....) will not lead to any layers being put in the vtile (needs test to confirm) whereas a test like this using vtile.addGeoJSON will create an empty layer (needs test to confirm).

/cc @ingalls

[springmeyer]

/cc @flippmoke - want to hit this in your v2 spec push (as I recall you said you saw cases of empty layers being added by mapnik-vt when they could otherwise be avoided since they had no features).