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

{
    switch (m_type) {
    case Type::ELISION: {
        return false;
    }
    case Type::ANY: {
        return true;
    }
    case Type::NONE: {
        return UniValue::VNULL == result.getType();
    }
    case Type::STR:
    case Type::STR_HEX: {
        return UniValue::VSTR == result.getType();
    }
    case Type::NUM:
    case Type::STR_AMOUNT:
    case Type::NUM_TIME: {
        return UniValue::VNUM == result.getType();
    }
    case Type::BOOL: {
        return UniValue::VBOOL == result.getType();
    }
    case Type::ARR_FIXED:
    case Type::ARR: {
        return UniValue::VARR == result.getType();
    }