Fixed a bug for fringe use-case

**Use-case**
> allowEmpty is true, field is submitted as empty and schema['default'] is null, a condition is added equal to null, which when combined with another field prevents results from being returned.

Which generates SQL such as ```LIKE '%Digital%' AND `Content`.`slug` IS NULL``` which won't match any results.

I think the problem was that `allowEmpty` was not being respected.