modified_utf7_codec library Null safety

Classes

ModifiedUtf7Codec
Provides Modified UTF7 encoder and decoder. Compare https://tools.ietf.org/html/rfc3501#section-5.1.3 and https://tools.ietf.org/html/rfc2152 for details. Inspired by https://github.com/jstedfast/MailKit/blob/master/MailKit/Net/Imap/ImapEncoding.cs