I'm sometimes getting an invalid json reply from `getwalletinfo` when rescanning is in progress.

The invalid part is this: `"scanning":{"duration":19,"progress":}}` (no value for `progress`).

I can't reliably reproduce this, but I did manage to run into this 3 times now (once every 10-20 attempts or so).

I'm running v0.20.0 and testing against a regtest instance. This happens while doing a batched `importmulti` of 2000 addresses (none of which have history).
std::vector<OutputGroup> groups;
    std::map<CTxDestination, OutputGroup> gmap;
    std::set<CTxDestination> full_groups;
    for (const auto& output : outputs) {
        if (output.fSpendable) {
            CTxDestination dst;
            CInputCoin input_coin = output.GetInputCoin();
            size_t ancestors, descendants;
            chain().getTransactionAncestry(output.tx->GetHash(), ancestors, descendants);
            if (!single_coin && ExtractDestination(output.tx->tx->vout[output.i].scriptPubKey, dst)) {
                auto it = gmap.find(dst);
                if (it != gmap.end()) {
                    
                    
                    
                    
                    
                    if (it->second.m_outputs.size() >= OUTPUT_GROUP_MAX_ENTRIES) {
                        groups.push_back(it->second);
                        it->second = OutputGroup{};
                        full_groups.insert(dst);
                    }
                    it->second.Insert(input_coin, output.nDepth, output.tx->IsFromMe(ISMINE_ALL), ancestors, descendants);
                } else {
                    gmap[dst].Insert(input_coin, output.nDepth, output.tx->IsFromMe(ISMINE_ALL), ancestors, descendants);
                }