x=NULL-Values are now projected as ISNULL(x) in some cases

NULL-Values used in conjunction with Select::where and Select::having when conditions are used as a key-value-array are now projected with ISNULL instead of field=NULL.

$str = $select
->field('a')
->from('t', 'test')
->where(['field' => null])
->asString();

will result in

SELECT
    a
FROM
    test t
WHERE
    (ISNULL(`field`))