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

const std::string name,
        const Type type,
        const Fallback fallback,
        const std::string description,
        const std::string oneline_description = "",
        const std::vector<std::string> type_str = {},
        const bool hidden = false)
        : m_names{std::move(name)},
          m_type{std::move(type)},
          m_hidden{hidden},
          m_fallback{std::move(fallback)},
          m_description{std::move(description)},
          m_oneline_description{std::move(oneline_description)},
          m_type_str{std::move(type_str)}
    {
        CHECK_NONFATAL(type != Type::ARR && type != Type::OBJ && type != Type::OBJ_USER_KEYS);
    }
    RPCArg(
        const std::string name,
        const Type type,
        const Fallback fallback,
        const std::string description,
        const std::vector<RPCArg> inner,
        const std::string oneline_description = "",
        const std::vector<std::string> type_str = {})
        : m_names{std::move(name)},
          m_type{std::move(type)},
          m_hidden{false},
          m_inner{std::move(inner)},