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

RPCResult{RPCResult::Type::NONE, "", ""},
                RPCExamples{
                    HelpExampleCli("keypoolrefill", "")
            + HelpExampleRpc("keypoolrefill", "")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return NullUniValue;
    if (pwallet->IsLegacy() && pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Private keys are disabled for this wallet");
    }
    LOCK(pwallet->cs_wallet);
    
    unsigned int kpSize = 0;
    if (!request.params[0].isNull()) {
        if (request.params[0].get_int() < 0)
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected valid size.");
        kpSize = (unsigned int)request.params[0].get_int();
    }
    EnsureWalletIsUnlocked(*pwallet);
    pwallet->TopUpKeyPool(kpSize);
    if (pwallet->GetKeyPoolSize() < kpSize) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error refreshing keypool.");
    }
    return NullUniValue;
},
    };
}
static RPCHelpMan walletpassphrase()