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

if (arg.m_fallback.index() == 2) {
            const RPCArg::Type type = arg.m_type;
            switch (std::get<RPCArg::Default>(arg.m_fallback).getType()) {
            case UniValue::VOBJ:
                CHECK_NONFATAL(type == RPCArg::Type::OBJ);
                break;
            case UniValue::VARR:
                CHECK_NONFATAL(type == RPCArg::Type::ARR);
                break;
            case UniValue::VSTR:
                CHECK_NONFATAL(type == RPCArg::Type::STR || type == RPCArg::Type::STR_HEX || type == RPCArg::Type::AMOUNT);
                break;
            case UniValue::VNUM:
                CHECK_NONFATAL(type == RPCArg::Type::NUM || type == RPCArg::Type::AMOUNT || type == RPCArg::Type::RANGE);
                break;
            case UniValue::VBOOL:
                CHECK_NONFATAL(type == RPCArg::Type::BOOL);
                break;
            case UniValue::VNULL:
                
                break;
            default:
                CHECK_NONFATAL(false);
                break;
            }
        }
    }
}
std::string RPCResults::ToDescriptionString() const