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).
{
    return strprintf("COutput(%s, %d, %d) [%s]", tx->GetHash().ToString(), i, nDepth, FormatMoney(tx->tx->vout[i].nValue));
}
const CWalletTx* CWallet::GetWalletTx(const uint256& hash) const
{
    AssertLockHeld(cs_wallet);
    std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(hash);
    if (it == mapWallet.end())
        return nullptr;
    return &(it->second);
}
void CWallet::UpgradeKeyMetadata()
{
    if (IsLocked() || IsWalletFlagSet(WALLET_FLAG_KEY_ORIGIN_METADATA)) {
        return;
    }
    auto spk_man = GetLegacyScriptPubKeyMan();
    if (!spk_man) {
        return;
    }
    spk_man->UpgradeKeyMetadata();
    SetWalletFlag(WALLET_FLAG_KEY_ORIGIN_METADATA);
}
bool CWallet::Unlock(const SecureString& strWalletPassphrase, bool accept_no_keys)