BOLT 12: Clarify handling of empty offer_chains field

[erickcestari]

The BOLT 12 specification doesn't explicitly address how to handle an offer_chains field that is present but contains zero chain hashes.

Currently the spec defines:

• Field omitted: Bitcoin only (implied)
• Field present with chains: Those specific chains
• Field present but empty: Undefined

I think we could reject such case or imply as "Bitcoin only"

Example offer: lno1qgqpvggrt0j7j3uzp9n549hxpu0sxlmpwe2ql5qplgwkg628wrzk5acfcskq

[thomash-acinq]

If the field is present but empty, it means that the offer is not valid for any chain.
From the Requirements for Invoice Requests:

if offer_chains is set:
MUST set invreq_chain to one of offer_chains

So if it's empty, you can't request an invoice.
That's how eclair treats it.