[SetupPayload] Set TLV buffer to null if there is no optional data

[vivien-apple]

Problem

While fixing #825 with PR #853 it introduced in the code a minimum size for the TLV buffer. Shana points out this is incorrect - but the code got already merged.

This PR fix that by setting not using a TLV buffer at all by default.

Also the change add a reference to the related paragraph of the specification. I have not seen any in the setup_payload code but I would like to add more since I think it would be helpful to match the current behaviour of the code to the related part of the spec.

Summary of Changes

• Use NULL instead of a buffer if there is no optional data
• Change the serial number test to use a 1 character serial number (0 got ignore by the underlying Implementation)
• Remove a leftover TODO

fixes #927