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::list<UniValueType>& typesExpected,
                  bool fAllowNull)
{
    unsigned int i = 0;
    for (const UniValueType& t : typesExpected) {
        if (params.size() <= i)
            break;
        const UniValue& v = params[i];
        if (!(fAllowNull && v.isNull())) {
            RPCTypeCheckArgument(v, t);
        }
        i++;
    }
}
void RPCTypeCheckArgument(const UniValue& value, const UniValueType& typeExpected)
{
    if (!typeExpected.typeAny && value.type() != typeExpected.type) {
        throw JSONRPCError(RPC_TYPE_ERROR, strprintf("Expected type %s, got %s", uvTypeName(typeExpected.type), uvTypeName(value.type())));
    }
}
void RPCTypeCheckObj(const UniValue& o,