mail: don't set an empty header field in SetMsgIDList
mail: don't set an empty header field in SetMsgIDList

Per RFC 5322:

    in-reply-to     =   "In-Reply-To:" 1*msg-id CRLF
    references      =   "References:" 1*msg-id CRLF

Message ID lists must contain at least 1 entry.

emersion/go-message