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).
{
    AssertLockHeld(cs_wallet);
    std::map<uint256, CWalletTx>::const_iterator mi = mapWallet.find(txin.prevout.hash);
    if (mi != mapWallet.end())
    {
        const CWalletTx& prev = (*mi).second;
        if (txin.prevout.n < prev.tx->vout.size())
            return IsMine(prev.tx->vout[txin.prevout.n]);
    }
    return ISMINE_NO;
}
CAmount CWallet::GetDebit(const CTxIn &txin, const isminefilter& filter) const
{
    {
        LOCK(cs_wallet);
        std::map<uint256, CWalletTx>::const_iterator mi = mapWallet.find(txin.prevout.hash);
        if (mi != mapWallet.end())
        {
            const CWalletTx& prev = (*mi).second;
            if (txin.prevout.n < prev.tx->vout.size())
                if (IsMine(prev.tx->vout[txin.prevout.n]) & filter)
                    return prev.tx->vout[txin.prevout.n].nValue;
        }
    }
    return 0;
}
isminetype CWallet::IsMine(const CTxOut& txout) const