OSCORE replay window serialization cleanup

[chrysn]

Currently, both "unknown" and {"index": null, "bitfield": null} are used and recognized expressions for the uninit'ed bitfield.

Compatible way forward:

• Only produce null for the received field (suggested new value by @miri64)
• Transitionally, accept any odd value (including the above) as unknown-state, and produce a warning, which ensures we don't actively produce that state any more (because if the tests produce warnings they fail)
• At some point, turn the warning into an error