Can not parse email with blank email address headers

For some incoming emails, it could be with header **Return-Path: <>**. This is possible for auto-replies emails.
This library always throws **MimeMessageParseException** when parsing those emails. 

[related code](https://github.com/bbottema/simple-java-mail/blob/master/src/main/java/org/simplejavamail/converter/internal/mimemessage/MimeMessageParser.java#L168)