Some message can have an _empty_ message-id, e.g. with:
In-Reply-To: <>
which we weren't filter out.
This would yield and _empty_ Thread-Id, in mu-message.cc
And this would make mu-query believe it had no matches in the first
query, in Query::Private::run_related, and effectively throw away the
results. (Xapian using empty string both for a "not found" result, and
"found an empty string doesn't help either).
So, avoid having an empty reference. Also add a unit-test.
Fixes#2812.
7baf1bf5e5