There are instances where a TLV IE is used as just a flag, i.e.
length zero and no value part. In those situations, it would require
a lot of boilerplate code to require the TLV_IE class definitions to
have _to_bytes/_from_bytes methods that do nothing.
So instead, add a shortcut: If we want to encode 'None', then return
b'', and if we want to decode b'' return None.
Change-Id: Ie8eb2830e8eefa81e94b8b8b157062c085aeb777