[Django] #36592: Incorrect warning when specifying a UniqueConstraint with SQLite and nulls_distinct=True

[Django]

#36592: Incorrect warning when specifying a UniqueConstraint with SQLite and
nulls_distinct=True
-------------------------------------+-------------------------------------
Reporter: r-owen | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 5.2 | Severity: Normal
Keywords: SQLite | Triage Stage:
UniqueConstraint | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
I am trying to create a UniqueConstraint with `nulls_distinct=False`. This
will be used with sqlite for local use and tests, and postgresql for
production.

When I migrate my sqlite database I get this warning:
```
SQLite does not support unique constraints with nulls distinct.
```
I suspect this is a bug, since I specified that nulls should not be
distinct.

Note that I also get the same warning if I specify `nulls_distinct=True`,
but in that case I expect to see it.

Here is the simplest model I could come up with that shows the issue:
```
class SimpleModel(models.Model):
field_a = models.IntegerField(null=True)
field_b = models.IntegerField(null=True)

class Meta:
constraints = [
UniqueConstraint(
name="simple_unique_constraint",
fields=["field_a", "field_b"],
nulls_distinct=False,
),
]
```
The full warning I see on migration is:
```
WARNINGS:
dances.SimpleModel: (models.W047) SQLite does not support unique
constraints with nulls distinct.
HINT: A constraint won't be created. Silence this warning if you
don't care about it.
```
and, as I said before, I get the same warning if I use
`nulls_distinct=True` (which is when I expect to see the warning).
--
Ticket URL: <https://code.djangoproject.com/ticket/36592>
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