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

{
    if (!fee_rate.isNull()) {
        if (!conf_target.isNull()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot specify both conf_target and fee_rate. Please provide either a confirmation target in blocks for automatic fee estimation, or an explicit fee rate.");
        }
        if (!estimate_mode.isNull() && estimate_mode.get_str() != "unset") {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot specify both estimate_mode and fee_rate");
        }
        cc.m_feerate = CFeeRate(AmountFromValue(fee_rate), COIN);
        if (override_min_fee) cc.fOverrideFeeRate = true;
        
        if (!cc.m_signal_bip125_rbf) cc.m_signal_bip125_rbf = true;
        return;
    }
    if (!estimate_mode.isNull() && !FeeModeFromString(estimate_mode.get_str(), cc.m_fee_mode)) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, InvalidEstimateModeErrorMessage());
    }
    if (!conf_target.isNull()) {
        cc.m_confirm_target = ParseConfirmTarget(conf_target, wallet.chain().estimateMaxBlocks());
    }
}
static RPCHelpMan getnewaddress()