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

+ HelpExampleCli("getbalance", "") +
            "\nThe total amount in the wallet with at least 6 confirmations\n"
            + HelpExampleCli("getbalance", "\"*\" 6") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("getbalance", "\"*\", 6")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return NullUniValue;
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    const UniValue& dummy_value = request.params[0];
    if (!dummy_value.isNull() && dummy_value.get_str() != "*") {
        throw JSONRPCError(RPC_METHOD_DEPRECATED, "dummy first argument must be excluded or set to \"*\".");
    }
    int min_depth = 0;
    if (!request.params[1].isNull()) {
        min_depth = request.params[1].get_int();
    }
    bool include_watchonly = ParseIncludeWatchonly(request.params[2], *pwallet);
    bool avoid_reuse = GetAvoidReuseFlag(*pwallet, request.params[3]);
    const auto bal = pwallet->GetBalance(min_depth, avoid_reuse);
    return ValueFromAmount(bal.m_mine_trusted + (include_watchonly ? bal.m_watchonly_trusted : 0));
},
    };
}
static RPCHelpMan getunconfirmedbalance()