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

[ptrcnull]

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.