Make <domain> in revServer *really* optional

[DL6ER]

What does this implement/fix?

Some time ago (in #2116), we fixed the config validator to accept revServer lines without <domain>, however, there was another check at the point where this configuration is written into dnsmasq.conf as users may have edited pihole.toml directly without going through the API (so, effectively, bypassing the validators). This last check still enforced <domain> which is relaxed herein.

Related issue or feature (if applicable): Fixes #2216

Pull request in docs with documentation (if applicable): N/A

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Checklist:

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[DL6ER]

Confirmed by OP in the referenced issue ticket

[pralor-bot]

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

https://discourse.pi-hole.net/t/v6-post-release-fixes-and-findings/76480/1