While some protocols require value attached to OP_RETURN outputs, this otherwise seems like a mistake.

Setting the value to 0 should really be done by the wallet/script creating the transaction (like createrawtransaction does for `data` outputs). However, as an additional check, similar to the high-fee check, sendrawtransaction could also check for non-zero value OP_RETURN outputs.

This should be configurable by a new RPC argument.

See also https://transactionfee.info/charts/output-opreturn-amount/

#### Useful skills:

* Bitcoin Core RPC code
* Transaction output types
* Bitcoin Core functional tests

#### Want to work on this issue?

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

{
    return RPCHelpMan{"gettxspendingprevout",
        "Scans the mempool to find transactions spending any of the given outputs",
        {
            {"outputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The transaction outputs that we want to check, and within each, the txid (string) vout (numeric).",
                {
                    {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                        {
                            {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                            {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                        },
                    },
                },
            },
        },
        RPCResult{
            RPCResult::Type::ARR, "", "",
            {
                {RPCResult::Type::OBJ, "", "",
                {
                    {RPCResult::Type::STR_HEX, "txid", "the transaction id of the checked output"},
                    {RPCResult::Type::NUM, "vout", "the vout value of the checked output"},
                    {RPCResult::Type::STR_HEX, "spendingtxid", true, "the transaction id of the mempool transaction spending this output (omitted if unspent)"},
                }},
            }
        },
        RPCExamples{
            HelpExampleCli("gettxspendingprevout", "\"[{\\\"txid\\\":\\\"a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0\\\",\\\"vout\\\":3}]\"")