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 (options.exists("minimumSumAmount"))
            nMinimumSumAmount = AmountFromValue(options["minimumSumAmount"]);
        if (options.exists("maximumCount"))
            nMaximumCount = options["maximumCount"].get_int64();
    }
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    UniValue results(UniValue::VARR);
    std::vector<COutput> vecOutputs;
    {
        CCoinControl cctl;
        cctl.m_avoid_address_reuse = false;
        cctl.m_min_depth = nMinDepth;
        cctl.m_max_depth = nMaxDepth;
        LOCK(pwallet->cs_wallet);
        pwallet->AvailableCoins(vecOutputs, !include_unsafe, &cctl, nMinimumAmount, nMaximumAmount, nMinimumSumAmount, nMaximumCount);
    }
    LOCK(pwallet->cs_wallet);
    const bool avoid_reuse = pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE);
    for (const COutput& out : vecOutputs) {
        CTxDestination address;
        const CScript& scriptPubKey = out.tx->tx->vout[out.i].scriptPubKey;
        bool fValidAddress = ExtractDestination(scriptPubKey, address);
        bool reused = avoid_reuse && pwallet->IsSpentKey(out.tx->GetHash(), out.i);
        if (destinations.size() && (!fValidAddress || !destinations.count(address)))
            continue;
        UniValue entry(UniValue::VOBJ);
        entry.pushKV("txid", out.tx->GetHash().GetHex());
        entry.pushKV("vout", out.i);