By using `select_related(None)` instead of `select_related()`, the
unnecessary joins are completely avoided. Note that the current tests
already covers the change, when the field is not `null=True`.
Regression in f9936deed1.
The __isnull lookup of JSONField must special case
Value(None, JSONField()) left-hand-side in order to be coherent with
its convoluted null handling.
Since psycopg>=3 offers no way to pass a NULL::jsonb the issue is
resolved by optimizing IsNull(Value(None), True | False) to
True | False.
Regression in 5c23d9f0c3.
Thanks Alexandre Collet for the report.
