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

flags |= WALLET_FLAG_EXTERNAL_SIGNER;
#else
        throw JSONRPCError(RPC_WALLET_ERROR, "Compiled without external signing support (required for external signing)");
#endif
    }
#ifndef USE_BDB
    if (!(flags & WALLET_FLAG_DESCRIPTORS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Compiled without bdb support (required for legacy wallets)");
    }
#endif
    DatabaseOptions options;
    DatabaseStatus status;
    options.require_create = true;
    options.create_flags = flags;
    options.create_passphrase = passphrase;
    bilingual_str error;
    std::optional<bool> load_on_start = request.params[6].isNull() ? std::nullopt : std::optional<bool>(request.params[6].get_bool());
    std::shared_ptr<CWallet> wallet = CreateWallet(*context.chain, request.params[0].get_str(), load_on_start, options, status, error, warnings);
    if (!wallet) {
        RPCErrorCode code = status == DatabaseStatus::FAILED_ENCRYPT ? RPC_WALLET_ENCRYPTION_FAILED : RPC_WALLET_ERROR;
        throw JSONRPCError(code, error.original);
    }
    UniValue obj(UniValue::VOBJ);
    obj.pushKV("name", wallet->GetName());
    obj.pushKV("warning", Join(warnings, Untranslated("\n")).original);
    return obj;
},
    };
}
static RPCHelpMan unloadwallet()