Add test for retrieving null dates

[kibertoad]

Looks like original issue was only with Maria dialect which we no longer support. This adds test from #1308 to make sure that other dialects don't have same problem.

fixes #1276

[kibertoad]

@wubzz Do you think that having these tests in knex to guard about possible regressions in the future makes sense?

[wubzz]

@kibertoad Yes, I think it's fine. The more coverage we have, the better. Things like these aren't obvious to add tests for generally speaking, you just expect it to work out of the box. But we've had situations where things we expect simply do not work, either due to knex or even on dialect level, so imho it's good additions.