Revert "[BIP322] remove empty message requirement for full (proof-of-…

…funds) p…"

This reverts commit 6fc75b1.

bip-0322.mediawiki

@@ -80,6 +80,8 @@ A full signature consists of the base64-encoding of the <code>to_sign</code> tra
 
 A signer may construct a proof of funds, demonstrating control of a set of UTXOs, by constructing a full signature as above, with the following modifications.
 
+* <code>message_challenge</code> is unused and shall be set to <code>OP_TRUE</code>
+* Similarly, <code>message_signature</code> is then empty.
 * All outputs that the signer wishes to demonstrate control of are included as additional inputs of <code>to_sign</code>, and their witness and scriptSig data should be set as though these outputs were actually being spent.
 
 Unlike an ordinary signature, validators of a proof of funds need access to the current UTXO set, to learn that the claimed inputs exist on the blockchain, and to learn their scriptPubKeys.