Remove null=True for DNSDomain and DNSRecords

[peterbppb]

dns_ips slack module assumes domains and records cannot be null so it tries to strip them here: https://github.com/surface-security/surface/blob/main/surface/dns_ips/slack.py#L74-83

We should either remove null=True to avoid this issue or filter for that in the slack processor.

I think removing null=True from these model fields makes sense as I do not think we have any need for records/domains without a name.

[gsilvapt]

I'm in favour of removing these attributes in both models, they are not appropriate for their data types.
When releasing this, we might need to add a note for all forks take into account this change (they will need to provide a default, most likely).