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

{
    return m_names.substr(0, m_names.find("|"));
}
std::string RPCArg::GetName() const
{
    CHECK_NONFATAL(std::string::npos == m_names.find("|"));
    return m_names;
}
bool RPCArg::IsOptional() const
{
    if (m_fallback.index() != 0) {
        return true;
    } else {
        return RPCArg::Optional::NO != std::get<RPCArg::Optional>(m_fallback);
    }
}
std::string RPCArg::ToDescriptionString() const
{
    std::string ret;
    ret += "(";
    if (m_type_str.size() != 0) {