Messages without a Content-Transfer-Encoding header pass an empty encoding to go-imap

as such, combined with https://github.com/emersion/go-imap/issues/545, the encoding gets serialized as `NIL` which is invalid according to spec; the encoding should default in this case to `7bit`:
> This is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the Content-Transfer-Encoding header field is not present.