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).
{
    const std::string& walletFile = database->Filename();
    chain.initMessage(_("Loading wallet...").translated);
    int64_t nStart = GetTimeMillis();
    bool fFirstRun = true;
    
    
    std::shared_ptr<CWallet> walletInstance(new CWallet(&chain, name, std::move(database)), ReleaseWallet);
    DBErrors nLoadWalletRet = walletInstance->LoadWallet(fFirstRun);
    if (nLoadWalletRet != DBErrors::LOAD_OK) {
        if (nLoadWalletRet == DBErrors::CORRUPT) {
            error = strprintf(_("Error loading %s: Wallet corrupted"), walletFile);
            return nullptr;
        }
        else if (nLoadWalletRet == DBErrors::NONCRITICAL_ERROR)
        {
            warnings.push_back(strprintf(_("Error reading %s! All keys read correctly, but transaction data"
                                           " or address book entries might be missing or incorrect."),
                walletFile));
        }
        else if (nLoadWalletRet == DBErrors::TOO_NEW) {
            error = strprintf(_("Error loading %s: Wallet requires newer version of %s"), walletFile, PACKAGE_NAME);
            return nullptr;
        }
        else if (nLoadWalletRet == DBErrors::NEED_REWRITE)
        {
            error = strprintf(_("Wallet needed to be rewritten: restart %s to complete"), PACKAGE_NAME);
            return nullptr;