[2.6] Unable to delete domain after adding A record with empty Data field.

Steps:

1. Go to DNS tab.
2. Create a new domain.
3. Click on new domain to go to details page.
4. Click on Add record, to add a new record.
5. Input some-name for Name field, choose A record for Type field, and leave Data and TTL empty.
6. Click on green Add record button.
7. Note that no new resource record shows up in the Resource records section towards the bottom of the page.
8. Click on Delete domain and again for confirmation and you will see error:

("Cannot delete some instances of model 'Domain' because they are referenced through a protected foreign key: 'DNSResource.domain'", <QuerySet [<DNSResource: name=something>]>)