[BUG] Carbon txt looks trigger integrity error from sitecheck IP ranges

[mrchrisadams]

Please choose the appropriate severity level accompanied by this report by removing the exess severity labels.

Describe the bug
When we do a lookup using the carbon.txt method, because we are using a domain based lookup instead of a IP based lookup, we are seeing integrity exceptions, because the table is expecting a number representing an IP address.

To Reproduce

Run a check on a domain where we only have carbon.txt related information.

Expected behavior

We should be able to successfully log these lookups, and pass an number of 0 as our IP in.

Actual behavior

The SiteCheckLogger will try to log a sitecheck that has a ip of None and an match_ip_range of None.

An integrity error (1048, "Column 'id_greencheck' cannot be null) exception stops the look up being logged successfully.

---

Carbon.txt isn't really an officially supported protocol so we see relatively few checks, and subsequently errors of this kind in our exception notifier.

Our handling of IP addresses for carbon.txt isn't something we have really defined that well for this table, and the Sitecheck code could do with some love anyway. This issue is resolved when we have:

• a clearly documented approach for handling carbon.txt lookups
• no exceptions triggered when logging them