fix(dao) handle ngx.null in C* constraints checks
fix(dao) handle ngx.null in C* constraints checks

We should not select with Cassandra `null` types for these constraints,
so while this change does not come with a reproducible test case, it is
still healthy to have.

See #3308
From #3355

Signed-off-by: Thibault Charbonnier <3c2e9133e272cb489e2fea0c4328cf56b08e4226@me.com>

Mashape/kong,Kong/kong,Kong/kong,Kong/kong