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

ret += "json array";
            break;
        }
        } 
    }
    if (m_fallback.index() == 1) {
        ret += ", optional, default=" + std::get<RPCArg::DefaultHint>(m_fallback);
    } else if (m_fallback.index() == 2) {
        ret += ", optional, default=" + std::get<RPCArg::Default>(m_fallback).write();
    } else {
        switch (std::get<RPCArg::Optional>(m_fallback)) {
        case RPCArg::Optional::OMITTED: {
            
            break;
        }
        case RPCArg::Optional::OMITTED_NAMED_ARG: {
            ret += ", optional"; 
            break;
        }
        case RPCArg::Optional::NO: {
            ret += ", required";
            break;
        }
        } 
    }
    ret += ")";
    ret += m_description.empty() ? "" : " " + m_description;
    return ret;
}
void RPCResult::ToSections(Sections& sections, const OuterType outer_type, const int current_indent) const