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).
{
    const auto& address_book_it = m_address_book.find(dest);
    if (address_book_it == m_address_book.end()) return nullptr;
    if ((!allow_change) && address_book_it->second.IsChange()) {
        return nullptr;
    }
    return &address_book_it->second;
}
bool CWallet::UpgradeWallet(int version, bilingual_str& error, std::vector<bilingual_str>& warnings)
{
    int prev_version = GetVersion();
    int nMaxVersion = version;
    if (nMaxVersion == 0) 
    {
        WalletLogPrintf("Performing wallet upgrade to %i\n", FEATURE_LATEST);
        nMaxVersion = FEATURE_LATEST;
        SetMinVersion(FEATURE_LATEST); 
    } else {
        WalletLogPrintf("Allowing wallet upgrade up to %i\n", nMaxVersion);
    }
    if (nMaxVersion < GetVersion())
    {
        error = _("Cannot downgrade wallet");
        return false;
    }
    SetMaxVersion(nMaxVersion);
    LOCK(cs_wallet);
    
    int max_version = GetVersion();
    if (!CanSupportFeature(FEATURE_HD_SPLIT) && max_version >= FEATURE_HD_SPLIT && max_version < FEATURE_PRE_SPLIT_KEYPOOL) {