docs(nut14): clarify pubkey-less HTLC spends on preimage alone

[robwoodgate]

Summary

Clarifies that an HTLC with no pubkeys tag is spendable by the preimage alone, with no signature required.

The Receiver Pathway currently says the receiver provides BOTH the preimage and a Locktime MultiSig signature, which reads as if a pubkeys tag is mandatory. But an HTLC locks to a hashlock in Secret.data (not a pubkey), so the pubkeys tag is optional — when it is absent there are no keys to sign and the preimage alone should spend the proof.

This adds a one-line **NOTE:** to the Receiver Pathway, mirroring the existing note on the Sender Pathway that covers the absent-refund/locktime cases.

Motivation

The absent-pubkeys case is currently undefined, so implementations diverge: the reference implementation (CDK) treats an empty pubkey list as a zero-signature receiver pathway and spends on the preimage alone, which is the behaviour this note documents.