mysql backend creates queries using CONVERT_TZ even when this is not supported

[Klaas van Schelven]

To test whether a mysql server has the zoneinfo database loaded (feature name: has_zoneinfo_database), the ​following code is run:

CONVERT_TZ('2001-01-01 01:00:00', 'UTC', 'UTC') IS NOT NULL

However, this test is not actually used (i.e. there is no condition on the feature being True) when ​generating sql, despite the fact that the generated sql contains something that we know will evaluate to NULL, namely, the very same CONVERT_TZ mentioned in the above.

This shows up when filtering like so:

Foo.objects(some_datetimefield__date="2023-06-16")

which evaluates to the empty queryset, despite there being such objects. USE_TZ = True, obviously