msgtime: assume +0000 if TZ missing when using Date::Parse

msgtime: assume +0000 if TZ missing when using Date::Parse

Some old emails don't have timezone offsets, since our
Date::Parse code path takes a liberal interpretation of dates,
fallback to using "+0000" as the timezone offset since it's
closer to the actual date of the message than whatever the
current date is.

Reported-by: Leah Neukirchen <leah@vuxu.org>
Link: https://public-inbox.org/meta/87h7zfemur.fsf@vuxu.org/
Fixes: ae80a3fdb53d7014 ("MsgTime.pm: Use strptime to compute the time zone")