Postgres queries fail when using `Submittable` interface

When capturing postgres is enabled queries made with the `Submittable` interface cause a TypeError indicating

> Client was passed a null or undefined query at Client.query

This makes it so the postgres capture does not work with [pg-cursor](https://github.com/brianc/node-postgres/tree/master/packages/pg-cursor) or [pg-query-stream](https://github.com/brianc/node-postgres/tree/master/packages/pg-query-stream).

The issue concerns PostgreSQL capture around Client.query; start by reproducing it with the Submittable interface, pg-cursor, and pg-query-stream. Done means those queries no longer pass a null or undefined query to Client.query and PostgreSQL capture works.

Written by the indexing model from the issue text.