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

{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return NullUniValue;
    LOCK(pwallet->cs_wallet);
    std::string purpose;
    if (!request.params[0].isNull()) {
        purpose = request.params[0].get_str();
    }
    
    std::set<std::string> label_set;
    for (const std::pair<const CTxDestination, CAddressBookData>& entry : pwallet->m_address_book) {
        if (entry.second.IsChange()) continue;
        if (purpose.empty() || entry.second.purpose == purpose) {
            label_set.insert(entry.second.GetLabel());
        }
    }
    UniValue ret(UniValue::VARR);
    for (const std::string& name : label_set) {
        ret.push_back(name);
    }
    return ret;
},
    };
}
static RPCHelpMan send()
{
    return RPCHelpMan{"send",
        "\nEXPERIMENTAL warning: this call may be changed in future releases.\n"
        "\nSend a transaction.\n",
        {