Handle the case where the domain or header domain is malformed

🗣 Description

This pull requests modifies the code to handle the case where the domain or header domain is malformed.

💭 Motivation and context

There was a case this past weekend where an HHS header domain was malformed such that the get_public_suffix() call failed because it was being passed a domain of None. With these changes the code will handle such cases without crashing and will print out a helpful logging message.

🧪 Testing

All pre-commit hooks pass. I also used these changes to successfully generate the HHS report from this past weekend.

✅ Pre-approval checklist

[x] This PR has an informative and human-readable title.
[x] Changes are limited to a single goal - eschew scope creep!
[x] All relevant type-of-change labels have been added.
[x] I have read the CONTRIBUTING document.
[x] These code changes follow cisagov code standards.
[x] All new and existing tests pass.

✅ Post-merge checklist

[x] Add a tag.

cisagov / trustymail_reporter