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

{"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
                    {"include_watching", RPCArg::Type::BOOL, RPCArg::DefaultHint{"true for watch-only wallets, otherwise false"}, "Also select inputs which are watch only.\n"
                                          "Only solvable inputs can be used. Watch-only destinations are solvable if the public key and/or output script was imported,\n"
                                          "e.g. with 'importpubkey' or 'importmulti' with the 'pubkeys' or 'desc' field."},
                    {"inputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Specify inputs instead of adding them automatically. A JSON array of JSON objects",
                        {
                            {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                            {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                            {"sequence", RPCArg::Type::NUM, RPCArg::Optional::NO, "The sequence number"},
                        },
                    },
                    {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
                    {"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                    {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                    {"subtract_fee_from_outputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Outputs to subtract the fee from, specified as integer indices.\n"
                    "The fee will be equally deducted from the amount of each specified output.\n"
                    "Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
                    "If no outputs are specified here, the sender pays the fee.",
                        {
                            {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                        },
                    },
                    {"replaceable", RPCArg::Type::BOOL, RPCArg::DefaultHint{"wallet default"}, "Marks this transaction as BIP125 replaceable.\n"
                                                  "Allows this transaction to be replaced by a transaction with higher fees"},
                },
                "options"},
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "",
                {