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

NO,
        /**
         * Optional arg that is a named argument and has a default value of
         * `null`. When possible, the default value should be specified.
         */
        OMITTED_NAMED_ARG,
        /**
         * Optional argument with default value omitted because they are
         * implicitly clear. That is, elements in an array or object may not
         * exist by default.
         * When possible, the default value should be specified.
         */
        OMITTED,
    };
    using DefaultHint = std::string;
    using Default = UniValue;
    using Fallback = std::variant<Optional,  DefaultHint,  Default>;
    const std::string m_names; 
    const Type m_type;
    const bool m_hidden;
    const std::vector<RPCArg> m_inner; 
    const Fallback m_fallback;
    const std::string m_description;
    const std::string m_oneline_description; 
    const std::vector<std::string> m_type_str; 
    RPCArg(