searchidx: deal with empty In-Reply-To and References headers
In some messages, these headers exist, but have empty values.
Do not let empty values throw off our search indexer to tie
threads together, as it can make non-sensical threads grouped
to a Message-Id of "" (empty string).
See
<https://public-inbox.org/git/
11340844841342-git-send-email-mailing-lists.git@rawuncut.elitemail.org/raw>
for an example of such a message.
Thanks-to: Johannes Schindelin <Johannes.Schindelin@gmx.de>
<https://public-inbox.org/git/alpine.DEB.2.20.
1702041206130.3496@virtualbox/>