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

#include <util/system.h>
#include <clientversion.h>
#include <hash.h> 
#include <key.h>  
#include <sync.h>
#include <test/util/logging.h>
#include <test/util/setup_common.h>
#include <test/util/str.h>
#include <uint256.h>
#include <util/getuniquepath.h>
#include <util/message.h> 
#include <util/moneystr.h>
#include <util/spanparsing.h>
#include <util/strencodings.h>
#include <util/string.h>
#include <util/time.h>
#include <util/vector.h>
#include <array>
#include <optional>
#include <stdint.h>
#include <string.h>
#include <thread>
#include <univalue.h>
#include <utility>
#include <vector>
#ifndef WIN32
#include <signal.h>