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).
{
    m_spk_man = pwallet->GetScriptPubKeyMan(type, internal);
    if (!m_spk_man) {
        return false;
    }
    if (nIndex == -1)
    {
        m_spk_man->TopUp();
        CKeyPool keypool;
        if (!m_spk_man->GetReservedDestination(type, internal, address, nIndex, keypool)) {
            return false;
        }
        fInternal = keypool.fInternal;
    }
    dest = address;
    return true;
}
void ReserveDestination::KeepDestination()
{
    if (nIndex != -1) {
        m_spk_man->KeepDestination(nIndex, type);
    }
    nIndex = -1;
    address = CNoDestination();
}
void ReserveDestination::ReturnDestination()