The NULL_DATE constant is defined conditionally in the DBA static class.
This causes issues with static analyzing tools like PHPStan, because
they can not really know if the constant is defined or not.
We could make PHPStan ignore this, but since there already is a
`get_null_date()` method on the `dba_driver` class, this patch
changes the code to use this method instead.
We could also use the public static attribute `$null_date` on the DBA
class directly, but using a method feels cleaner, and allows for making
the attribute private, or even removing it completely at some later
time.
I'm not removing the NULL_DATE constant for now, in case it is in use by
any extensions.
