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

bool fStrict = false);
/**
 * Utilities: convert hex-encoded Values
 * (throws error if not hex).
 */
extern uint256 ParseHashV(const UniValue& v, std::string strName);
extern uint256 ParseHashO(const UniValue& o, std::string strKey);
extern std::vector<unsigned char> ParseHexV(const UniValue& v, std::string strName);
extern std::vector<unsigned char> ParseHexO(const UniValue& o, std::string strKey);
extern CAmount AmountFromValue(const UniValue& value);
using RPCArgList = std::vector<std::pair<std::string, UniValue>>;
extern std::string HelpExampleCli(const std::string& methodname, const std::string& args);
extern std::string HelpExampleCliNamed(const std::string& methodname, const RPCArgList& args);
extern std::string HelpExampleRpc(const std::string& methodname, const std::string& args);
extern std::string HelpExampleRpcNamed(const std::string& methodname, const RPCArgList& args);
CPubKey HexToPubKey(const std::string& hex_in);
CPubKey AddrToPubKey(const FillableSigningProvider& keystore, const std::string& addr_in);
CTxDestination AddAndGetMultisigDestination(const int required, const std::vector<CPubKey>& pubkeys, OutputType type, FillableSigningProvider& keystore, CScript& script_out);
UniValue DescribeAddress(const CTxDestination& dest);
unsigned int ParseConfirmTarget(const UniValue& value, unsigned int max_target);
RPCErrorCode RPCErrorFromTransactionError(TransactionError terr);
UniValue JSONRPCTransactionError(TransactionError terr, const std::string& err_string = "");
std::pair<int64_t, int64_t> ParseDescriptorRange(const UniValue& value);
std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, FlatSigningProvider& provider);
UniValue GetServicesNames(ServiceFlags services);
/**
 * Serializing JSON objects depends on the outer type. Only arrays and
 * dictionaries can be nested in json. The top-level outer type is "NONE".
 */
enum class OuterType {