Need to add a null check in NS validate
File "dnstats/celery.py", line 231, in grade_ns
grade, errors = grade_ns_records(records, ip_addresses, record_results, site.domain)
File "dnstats/grading/ns.py", line 6, in grade
ns = Ns(ns_records, ns_ip_addresses, ns_record_results, domain)
File "dnstats/dnsvalidate/ns.py", line 34, in __init__
self._result = self._validate()
File "dnstats/dnsvalidate/ns.py", line 68, in _validate
for ns_result in self.ns_server_ns_results[ns_server]:```