the nutzap event containing P2PK-locked proofs
the single pubkey every proof is locked to, or undefined if there is no such pubkey — no proofs, a proof that is not P2PK locked, or proofs locked to more than one pubkey. A nutzap arrives from a stranger, so all three are hostile input rather than programmer errors, and none of them throw. This matches getProofP2PKPubkey one level down, which already returns undefined for every failure.
Extracts the P2PK locking pubkey from proofs in a nutzap event