Check for null in where clauses

[danvk]

If the where clause of a query contains a null value, or is a set containing a null value, then we replace:

• WHERE column = $1 → WHERE (column IS NULL OR column = $1)
• WHERE column = ANY($1) → WHERE (column IS NULL OR column = ANY($1))

Especially in the former case this is a little roundabout since the column = $1 clause can never match. But this does have the advantage that we don't need to renumber all the remaining fields.

Fixes #22