bpo-44852: Support filtering over warnings without a set message (GH-27793)

Additional improvements:

- messages which were compiled regular expressions aren't unpacked back into
  strings for unmatched warnings;

- removed unnecessary "if tokens:" check (there's one before the for loop);

- took `endswith` calculation out of the for loop.
(cherry picked from commit 8cf07d3db3eed02b43350a5f9dbf68f1c839ea82)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>