Disable async crypt for mails without subject

* src/mail.cpp (Mail::check_inline_response): Disable also
for empty subject.

For mails with an empty subject Outlook asks if the mail
should be sent anyway. If we do async crypto it also asks
on our triggered send event. In that case the overlay
blocks it. So better just disable async in that case so
that there are no two questions.

• GnuPG-Bug-Id: T4150