bulk_update() on JSONField saves JSON null instead of SQL NULL

[Adam Johnson]

I have a client project with a Django model that has a JSONField and a check constraint requiring the field to either contain SQL NULL or a JSON object (via a custom database function calling PostgreSQL's jsonb_typeof). Upgrading to Django 5.2 causes a bulk_update() query on that model to start failing with IntegrityError, because the JSON null is serialized instead ('null'::json in PostgreSQL syntax).

Like #36404 and #36405, this is another bug that bisects to e306687a3a5507d59365ba9bf545010e5fd4b2a8.

The issue:

1. BaseExpression.resolve_expression() no longer passes the for_save parameter through to source expressions.
2. When a source expression is a Value, for_save being False prevents it from calling Field.get_db_prep_save(): ​https://github.com/django/django/blob/f0a87895ffaf6532a22143b5e2e304c59b7958ae/django/db/models/expressions.py#L1170-L1173 .
3. JSONField.get_db_prep_save() has specific behaviour for None, ensuring it's returned as-is rather than wrapped as JSON: ​https://github.com/django/django/blob/main/django/db/models/fields/json.py#L110-L111
4. The missed call means None is wrapped as JSON null, which fails the model's check constraint.

Patch incoming.

[Jacob Walls]

Duplicate of #36419