JSD mail handler fails to process if incoming mail has an empty subject line

JSD mail handler fails to process if incoming mail has an empty subject line and the processing register log says "The subject of this mail is empty".

As this mail usually contains a valid issue, described in the mail body, and the customer doesn't receive any processing failure, the customer remains unattended.

To fill the missing subject line with, i.e, "[EMPTY SUBJECT]" and thus allowing issue creation and customer assistance.