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).
{
    bool is_mine;
    WalletBatch batch(*database);
    {
        LOCK(cs_wallet);
        
        
        
        if (IsMine(address)) {
            WalletLogPrintf("%s called with IsMine address, NOT SUPPORTED. Please report this bug! %s\n", __func__, PACKAGE_BUGREPORT);
            return false;
        }
        
        std::string strAddress = EncodeDestination(address);
        for (const std::pair<const std::string, std::string> &item : m_address_book[address].destdata)
        {
            batch.EraseDestData(strAddress, item.first);
        }
        m_address_book.erase(address);
        is_mine = IsMine(address) != ISMINE_NO;
    }
    NotifyAddressBookChanged(this, address, "", is_mine, "", CT_DELETED);
    batch.ErasePurpose(EncodeDestination(address));
    return batch.EraseName(EncodeDestination(address));
}
size_t CWallet::KeypoolCountExternalKeys() const