If you do an "all-text" search for a word, that
produces no match, a mysql error will be provoked.

mysql doesn't like this query:

    SELECT _issue.id
    FROM _issue
    WHERE _issue.__retired__ <>1 AND _issue.id
    IN ()

which is produced in back_mysql.filter():
the indexer returns search_matches = {} and the backend
checks:

    if search_matches is not None:

which obviously doesn't work.

I thought that:

     if not search_matches:
         ...

would catch both None and {}, but this didn't work. See
patch for my clumsy solution