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 (pwallet == nullptr)
        return 0;
    
    bool allow_cache = (filter & ISMINE_ALL) && (filter & ISMINE_ALL) != ISMINE_ALL;
    
    if (IsImmatureCoinBase())
        return 0;
    if (fUseCache && allow_cache && m_amounts[AVAILABLE_CREDIT].m_cached[filter]) {
        return m_amounts[AVAILABLE_CREDIT].m_value[filter];
    }
    bool allow_used_addresses = (filter & ISMINE_USED) || !pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE);
    CAmount nCredit = 0;
    uint256 hashTx = GetHash();
    for (unsigned int i = 0; i < tx->vout.size(); i++)
    {
        if (!pwallet->IsSpent(hashTx, i) && (allow_used_addresses || !pwallet->IsSpentKey(hashTx, i))) {
            const CTxOut &txout = tx->vout[i];
            nCredit += pwallet->GetCredit(txout, filter);
            if (!MoneyRange(nCredit))
                throw std::runtime_error(std::string(__func__) + " : value out of range");
        }
    }
    if (allow_cache) {
        m_amounts[AVAILABLE_CREDIT].Set(filter, nCredit);
        m_is_cache_empty = false;
    }
    return nCredit;
}
CAmount CWalletTx::GetImmatureWatchOnlyCredit(const bool fUseCache) const