Revert empty domain dot replacement

[yubiuser]

What does this PR aim to accomplish?:

Reverts #1244. This was introduced to replace "empty" domain strings with a dot (.). For historic reasons, dnsmasq strippes all trailing dots. If a query consists only of the dot (root zone), FTL has been seeing only an empty domain so far. FTL PR pi-hole/FTL#1413 changes this behavior, therefore, the mock-up introduced by #1244 is no longer needed.

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[DL6ER]

You're also removing this from db_queries.js - this may not be a good idea in case users are looking back in time until before the FTL update.

[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

[yubiuser]

I added special handling for the root zone during domain validation and excepted it from IDNA conversion (it always failed the conversion, returning false and preventing adding the root zone to the database)