parser: fix parsing of messages with empty subjects

parser: fix parsing of messages with empty subjects

The fuzz fixups made the test too strict ("if not subject" rather than
"if subject is None") in an attempt to catch broken subject headers.
This broke parsing of messages with an empty subject.

Fix it and add a test.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>