pg: Fix { $val: null } without $put: true

[aravindet]

Background:

Previously, this combination did not work (it would delete the property as if null was written, not { $val: null }). This behavior was inconsistent with Graffy semantics.

Changes:

The root cause of the bug was the use of the Postgres jsonb_strip_nulls() function, which recursively removed nulls; the behavior we wanted was a shallow version of this.

As no built-in function exists with the behavior we want, uses of jsonb_strip_nulls was replaced with the following subquery:

select jsonb_object_agg(key, value) from jsonb_each($object) where value <> 'null'::jsonb)

where $object is an expression returning a JSON object whose null properties are to be removed.

Additional changes to simplify the generated SQL:

• skip the above null-stripping subquery where we are certain that the object cannot have any null properties
• skip the pre-existing nullif() function (which converts an empty object to null), where we are certain that the object cannot be empty.