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.

{
    if (verbose) {
        if (include_mempool_sequence) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Verbose results cannot contain mempool sequence values.");
        }
        LOCK(pool.cs);
        UniValue o(UniValue::VOBJ);
        for (const CTxMemPoolEntry& e : pool.mapTx) {
            const uint256& hash = e.GetTx().GetHash();
            UniValue info(UniValue::VOBJ);
            entryToJSON(pool, info, e);
            
            
            
            o.__pushKV(hash.ToString(), info);
        }
        return o;
    } else {
        uint64_t mempool_sequence;
        std::vector<uint256> vtxid;
        {
            LOCK(pool.cs);
            pool.queryHashes(vtxid);
            mempool_sequence = pool.GetSequence();
        }
        UniValue a(UniValue::VARR);