Make a single column non-optional by filtering missing value rows

[virusdave]

Will be a test for a more advanced version of this library.

Currently, we can turn a Query[Rep[Option[A], ...] into a Query[Rep[A], ...] pretty easily if we can supply a default A, by doing:
query.map(maybeA => maybeA.getOrElse(defaultA)).

We should be able to lift this to arbitrary product-type rows.
We should also be able to do this without supplying a default A by filtering away all rows that are missing an A.