Add "ABSENT ON NULL" to JsonArray

[john-parton]

On all backends except Sqlite and Postgres version 15 and earlier, there is an additional clause to the JSON_ARRAY SQL which controls whether SQL NULL values are mapped to JSON null values or whether they are omitted.

In sqlite and postgres 15 and earlier, the default behavior is to map them, so my initial feature for JsonArray just had that as the only behavior without any ability to customize on the richer backends.

It would be nice if there was a simple on_null parameter to JSON ARRAY to control the behavior.

Pull request here: ​https://github.com/django/django/pull/19097

[john-parton]

Pull Request here: ​​https://github.com/django/django/pull/19097

[Simon Charette]

What do to of nulls was a point of discussion when the JSONArray feature was added so supporting either mode, when the backends allows, seems like a valuable feature.

FWIW it's also something that was discussed in the design of JSONArrayAgg #35462.

[john-parton]

I think my solution is pretty straightforward, actually. if we'd rather centralize discussion there, we can. I think the only real coordination would be on the feature bool.

[john-parton]

Note: My solution is only sort of straightforward now. I've added a workaround to make MySQL and older versions of Postgres behave.