Fixes OID warnings when using PostgreSQL

Postgres correctly identifies OIDs when `BEARING` and `DISTANCE` columns contain numerical values. It fails to identify OID for NULL when it expects a numerical value. That results in these two warnings:

    unknown OID: distance(705) (SELECT  cities.*, NULL AS distance, NULL AS bearing FROM "cities"  WHERE (false)  ORDER BY "cities"."id" ASC LIMIT 1)
    unknown OID: bearing(705) (SELECT  cities.*, NULL AS distance, NULL AS bearing FROM "cities"  WHERE (false)  ORDER BY "cities"."id" ASC LIMIT 1)

This patch explicitly sets an OID type `::text` for NULLs when running in PostgreSQL environment. `::text` is a default OID type in PostgreSQL.

References #637 and #628