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).
if (nFeePerK > HIGH_TX_FEE_PER_KB) {
            warnings.push_back(AmountHighWarn("-fallbackfee") + Untranslated(" ") +
                               _("This is the transaction fee you may pay when fee estimates are not available."));
        }
        walletInstance->m_fallback_fee = CFeeRate(nFeePerK);
    }
    
    walletInstance->m_allow_fallback_fee = walletInstance->m_fallback_fee.GetFeePerK() != 0;
    if (gArgs.IsArgSet("-discardfee")) {
        CAmount nFeePerK = 0;
        if (!ParseMoney(gArgs.GetArg("-discardfee", ""), nFeePerK)) {
            error = strprintf(_("Invalid amount for -discardfee=<amount>: '%s'"), gArgs.GetArg("-discardfee", ""));
            return nullptr;
        }
        if (nFeePerK > HIGH_TX_FEE_PER_KB) {
            warnings.push_back(AmountHighWarn("-discardfee") + Untranslated(" ") +
                               _("This is the transaction fee you may discard if change is smaller than dust at this level"));
        }
        walletInstance->m_discard_rate = CFeeRate(nFeePerK);
    }
    if (gArgs.IsArgSet("-paytxfee")) {
        CAmount nFeePerK = 0;
        if (!ParseMoney(gArgs.GetArg("-paytxfee", ""), nFeePerK)) {
            error = AmountErrMsg("paytxfee", gArgs.GetArg("-paytxfee", ""));
            return nullptr;
        }
        if (nFeePerK > HIGH_TX_FEE_PER_KB) {
            warnings.push_back(AmountHighWarn("-paytxfee") + Untranslated(" ") +
                               _("This is the transaction fee you will pay if you send a transaction."));
        }