Substitute "." for empty domain names

[DL6ER]

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Substitute "." from empty domain names passed by dnsmasq. This PR extends the already existing facility from internally generated DNSSEC queries also to regular queries.

[yubiuser]

Works as expected. Benefit is also that queries for . are now also stored in the long-term database. They have been stored before as well (but of course as empty domain)

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/regex-for-blocking-just-dot-as-domain/57319/13