Bugfix: getBaseDomain should return null when invalid

[englehardt]

This fixes a regression in the ad domain detection where we expect getBaseDomain to return null when a domain provided by SERP is unparsable, but getBaseDomain will always return the hostname (even if it's nonsensical).

This is Ad 11 and Add 12 in https://www.search-company.site/

I also fixed a no-op test that we forgot to update when we first merged the bat.js work. I only detected it because I had an outdated privacy grade reference that was still returning firstParty when running the tests locally.

Reviewer: @jonathanKingston

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[jonathanKingston]

Thanks for this!