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).
{
    LOCK(cs_wallet);
    std::set<CTxDestination> result;
    for (const std::pair<const CTxDestination, CAddressBookData>& item : m_address_book)
    {
        if (item.second.IsChange()) continue;
        const CTxDestination& address = item.first;
        const std::string& strName = item.second.GetLabel();
        if (strName == label)
            result.insert(address);
    }
    return result;
}
bool ReserveDestination::GetReservedDestination(CTxDestination& dest, bool internal)