values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

RPCExamples{
                            "\nCreate a transaction with no inputs\n"
                            + HelpExampleCli("createrawtransaction", "\"[]\" \"{\\\"myaddress\\\":0.01}\"") +
                            "\nAdd sufficient unsigned inputs to meet the output value\n"
                            + HelpExampleCli("fundrawtransaction", "\"rawtransactionhex\"") +
                            "\nSign the transaction\n"
                            + HelpExampleCli("signrawtransactionwithwallet", "\"fundedtransactionhex\"") +
                            "\nSend the transaction\n"
                            + HelpExampleCli("sendrawtransaction", "\"signedtransactionhex\"")
                                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return NullUniValue;
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValueType(), UniValue::VBOOL});
    
    CMutableTransaction tx;
    bool try_witness = request.params[2].isNull() ? true : request.params[2].get_bool();
    bool try_no_witness = request.params[2].isNull() ? true : !request.params[2].get_bool();
    if (!DecodeHexTx(tx, request.params[0].get_str(), try_no_witness, try_witness)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    }
    CAmount fee;
    int change_position;
    CCoinControl coin_control;
    
    coin_control.m_add_inputs = true;
    FundTransaction(*pwallet, tx, fee, change_position, request.params[1], coin_control,  true);
    UniValue result(UniValue::VOBJ);
    result.pushKV("hex", EncodeHexTx(CTransaction(tx)));