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

{
    
    
    wallet.BlockUntilSyncedToCurrentChain();
    change_position = -1;
    bool lockUnspents = false;
    UniValue subtractFeeFromOutputs;
    std::set<int> setSubtractFeeFromOutputs;
    if (!options.isNull()) {
      if (options.type() == UniValue::VBOOL) {
        
        coinControl.fAllowWatchOnly = options.get_bool();
      }
      else {
        RPCTypeCheckArgument(options, UniValue::VOBJ);
        RPCTypeCheckObj(options,
            {
                {"add_inputs", UniValueType(UniValue::VBOOL)},
                {"add_to_wallet", UniValueType(UniValue::VBOOL)},
                {"changeAddress", UniValueType(UniValue::VSTR)},
                {"change_address", UniValueType(UniValue::VSTR)},
                {"changePosition", UniValueType(UniValue::VNUM)},
                {"change_position", UniValueType(UniValue::VNUM)},