optimise `json_get(...) is null` to `json_contains_not_null(...)`

We can't replace json_get(x, y) is null with NOT json_contains(x, y) because

select json_get('{"foo": null}', 'foo') is null  -- true
select not json_contains('{"foo": null}', 'foo')  -- false
-- or equivilantly
select json_get('{"foo": null}', 'foo') is not null  -- false
select json_contains('{"foo": null}', 'foo')  -- true

So we need a new method json_contains_not_null which is true if the value exists in the JSON, but is not null.

Then we can do the substitution.

Start by locating the existing json_get and json_contains implementations and their surrounding function registration or dispatch code. Compare their handling of JSON null values, then verify that json_contains_not_null distinguishes an existing null from a missing value and supports the intended substitution.