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);
        for (std::pair<const uint256, CWalletTx>& item : mapWallet)
            item.second.MarkDirty();
    }
}
bool CWallet::MarkReplaced(const uint256& originalHash, const uint256& newHash)
{
    LOCK(cs_wallet);
    auto mi = mapWallet.find(originalHash);
    
    assert(mi != mapWallet.end());
    CWalletTx& wtx = (*mi).second;
    
    assert(wtx.mapValue.count("replaced_by_txid") == 0);
    wtx.mapValue["replaced_by_txid"] = newHash.ToString();
    WalletBatch batch(*database);
    bool success = true;
    if (!batch.WriteTx(wtx)) {
        WalletLogPrintf("%s: Updating batch tx %s failed\n", __func__, wtx.GetHash().ToString());
        success = false;
    }
    NotifyTransactionChanged(this, originalHash, CT_UPDATED);
    return success;
}
void CWallet::SetSpentKeyState(WalletBatch& batch, const uint256& hash, unsigned int n, bool used, std::set<CTxDestination>& tx_destinations)