Add support for non-distinct NULL expressions to UniqueConstraint.validate()

[Mark Gensler]

Expressions which evaluate to NULL within UniqueConstraint(*expressions, nulls_distinct=False) are still treated as distinct by UniqueConstraint.validate(). This means a ValidationError is not raised when it should be.

Similarly, if the database connection uses interprets_empty_strings_as_nulls this is also ignored by .validate().

Should #35575 be merged, this problem would also extend to any GeneratedField included in UniqueConstraint(fields=[...], nulls_distinct=False).

E.g.

class Book(models.Model):
    name = CharField(max_length=255, null=True, blank=True)

    class Meta:
        constraints = [
            UniqueConstraint(F("name"), nulls_distinct=False, name="book_name_null_unique")
        ]

then

> Book.objects.create(name=None)
> book = Book(name=None)
> book.full_clean() # Should raise a `ValidationError` but doesn't.
> book.save()  # The database raises a `UniqueViolation`.

This ticket was raised following discussion in ​https://github.com/django/django/pull/18356#pullrequestreview-2166340541