Bug 32575: Add an empty text body to fix multipart/mixed handling

Bug 32575: Add an empty text body to fix multipart/mixed handling

By adding an empty text body, we force Email::Stuffer/Email::MIME
to use multipart/mixed handling for the attachment instead of forcing
a single part (ie direct attachment) email, which is not consistently
handled by different email clients.

An empty text body is language-neutral (ie not imposing English),
and it allows SMTP servers to inject organisational footers into
the email (e.g. confidentiality notices).

Signed-off-by: Magnus Enger <magnus@libriotech.no>
I have tested this solution in production, and it works for me.

Bug 32575: Tidy patch

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>