Add whereNull and whereNotNull

[vesper8]

Describe the feature

Since you want to be closer to Laravel Eloquent's ORM feature set, it would be very useful to have whereNull and whereNotNull

Additional information

• Would you be willing to help implement this feature?

Final checks

• Check existing discussions and issues.

[vesper8]

@CodeDredd along the same lines, could you please also add whereNotIn / orWhereIn / orWhereNotIn