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).
{
    auto spk_man = GetLegacyScriptPubKeyMan();
    if (!spk_man) {
        return false;
    }
    LOCK(spk_man->cs_KeyStore);
    if (!spk_man->ImportScriptPubKeys(script_pub_keys, have_solving_data, timestamp)) {
        return false;
    }
    if (apply_label) {
        WalletBatch batch(*database);
        for (const CScript& script : script_pub_keys) {
            CTxDestination dest;
            ExtractDestination(script, dest);
            if (IsValidDestination(dest)) {
                SetAddressBookWithDB(batch, dest, label, "receive");
            }
        }
    }
    return true;
}
int64_t CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, bool use_max_sig)