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

{
    return RPCHelpMan{"upgradewallet",
        "\nUpgrade the wallet. Upgrades to the latest version if no version number is specified.\n"
        "New keys may be generated and a new wallet backup will need to be made.",
        {
            {"version", RPCArg::Type::NUM, RPCArg::Default{FEATURE_LATEST}, "The version number to upgrade to. Default is the latest wallet version."}
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "",
            {
                {RPCResult::Type::STR, "wallet_name", "Name of wallet this operation was performed on"},
                {RPCResult::Type::NUM, "previous_version", "Version of wallet before this operation"},
                {RPCResult::Type::NUM, "current_version", "Version of wallet after this operation"},
                {RPCResult::Type::STR, "result",  true, "Description of result, if no error"},
                {RPCResult::Type::STR, "error",  true, "Error message (if there is one)"}
            },
        },
        RPCExamples{
            HelpExampleCli("upgradewallet", "169900")
            + HelpExampleRpc("upgradewallet", "169900")
        },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return NullUniValue;
    RPCTypeCheck(request.params, {UniValue::VNUM}, true);
    EnsureWalletIsUnlocked(*pwallet);
    int version = 0;
    if (!request.params[0].isNull()) {