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).
{
    Balance ret;
    isminefilter reuse_filter = avoid_reuse ? ISMINE_NO : ISMINE_USED;
    {
        LOCK(cs_wallet);
        std::set<uint256> trusted_parents;
        for (const auto& entry : mapWallet)
        {
            const CWalletTx& wtx = entry.second;
            const bool is_trusted{IsTrusted(wtx, trusted_parents)};
            const int tx_depth{wtx.GetDepthInMainChain()};
            const CAmount tx_credit_mine{wtx.GetAvailableCredit( true, ISMINE_SPENDABLE | reuse_filter)};
            const CAmount tx_credit_watchonly{wtx.GetAvailableCredit( true, ISMINE_WATCH_ONLY | reuse_filter)};
            if (is_trusted && tx_depth >= min_depth) {
                ret.m_mine_trusted += tx_credit_mine;
                ret.m_watchonly_trusted += tx_credit_watchonly;
            }
            if (!is_trusted && tx_depth == 0 && wtx.InMempool()) {
                ret.m_mine_untrusted_pending += tx_credit_mine;
                ret.m_watchonly_untrusted_pending += tx_credit_watchonly;
            }
            ret.m_mine_immature += wtx.GetImmatureCredit();
            ret.m_watchonly_immature += wtx.GetImmatureWatchOnlyCredit();
        }
    }
    return ret;
}
CAmount CWallet::GetAvailableBalance(const CCoinControl* coinControl) const