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

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{
    return RPCHelpMan{"getmempoolentry",
        "\nReturns mempool data for given transaction\n",
        {
            {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id (must be in mempool)"},
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "", MempoolEntryDescription()},
        RPCExamples{
            HelpExampleCli("getmempoolentry", "\"mytxid\"")
            + HelpExampleRpc("getmempoolentry", "\"mytxid\"")
        },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    uint256 hash = ParseHashV(request.params[0], "parameter 1");
    const CTxMemPool& mempool = EnsureAnyMemPool(request.context);
    LOCK(mempool.cs);
    CTxMemPool::txiter it = mempool.mapTx.find(hash);
    if (it == mempool.mapTx.end()) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Transaction not in mempool");
    }
    const CTxMemPoolEntry &e = *it;
    UniValue info(UniValue::VOBJ);
    entryToJSON(mempool, info, e);
    return info;
},
    };
}
static RPCHelpMan gettxspendingprevout()