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?

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OK, 
    MISSING_INPUTS,
    ALREADY_IN_CHAIN,
    P2P_DISABLED,
    MEMPOOL_REJECTED,
    MEMPOOL_ERROR,
    INVALID_PSBT,
    PSBT_MISMATCH,
    SIGHASH_MISMATCH,
    MAX_FEE_EXCEEDED,
    EXTERNAL_SIGNER_NOT_FOUND,
    EXTERNAL_SIGNER_FAILED,
    INVALID_PACKAGE,
};
bilingual_str TransactionErrorString(const TransactionError error);
bilingual_str ResolveErrMsg(const std::string& optname, const std::string& strBind);
bilingual_str InvalidPortErrMsg(const std::string& optname, const std::string& strPort);
bilingual_str AmountHighWarn(const std::string& optname);
bilingual_str AmountErrMsg(const std::string& optname, const std::string& strValue);
#endif