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);
    error.clear();
    bool result = false;
    auto spk_man = GetScriptPubKeyMan(type, false );
    if (spk_man) {
        spk_man->TopUp();
        result = spk_man->GetNewDestination(type, dest, error);
    } else {
        error = strprintf("Error: No %s addresses available.", FormatOutputType(type));
    }
    if (result) {
        SetAddressBook(dest, label, "receive");
    }
    return result;
}
bool CWallet::GetNewChangeDestination(const OutputType type, CTxDestination& dest, std::string& error)
{
    LOCK(cs_wallet);
    error.clear();
    ReserveDestination reservedest(this, type);
    if (!reservedest.GetReservedDestination(dest, true)) {
        error = _("Error: Keypool ran out, please call keypoolrefill first").translated;
        return false;
    }
    reservedest.KeepDestination();
    return true;
}
int64_t CWallet::GetOldestKeyPoolTime() const