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);
    if (nWalletVersion >= nVersion)
        return;
    
    if (fExplicit && nVersion > nWalletMaxVersion)
            nVersion = FEATURE_LATEST;
    nWalletVersion = nVersion;
    if (nVersion > nWalletMaxVersion)
        nWalletMaxVersion = nVersion;
    {
        WalletBatch* batch = batch_in ? batch_in : new WalletBatch(*database);
        if (nWalletVersion > 40000)
            batch->WriteMinVersion(nWalletVersion);
        if (!batch_in)
            delete batch;
    }
}
bool CWallet::SetMaxVersion(int nVersion)
{
    LOCK(cs_wallet);
    
    if (nWalletVersion > nVersion)
        return false;
    nWalletMaxVersion = nVersion;
    return true;
}
std::set<uint256> CWallet::GetConflicts(const uint256& txid) const