Fix missing empty line after headers

[ytti]

Between email headers and body there wasnt empty line, but there was empty string in front of the body.

[skredder]

Would be great to merge this as strict MIME checking in some spamfilters is blocking SQA mails
(eg: https://forum.proxmox.com/threads/proxmox-mail-gateway-security-advisories.149333/#post-838667)

Subject: RING ALERT node.ring.nlnog.net: clearing ipv4 alarm
List-Id: ring-sqa <sqa.ring.nlnog.net>
X-Mailer: ring-sqa
node.ring.nlnog.net: clearing ipv4 alarm      <- body parsed as a header
Message-Id: <...>                              <- added by the MTA afterwards