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).
std::list<COutputEntry>& listSent, CAmount& nFee, const isminefilter& filter) const
{
    nFee = 0;
    listReceived.clear();
    listSent.clear();
    
    CAmount nDebit = GetDebit(filter);
    if (nDebit > 0) 
    {
        CAmount nValueOut = tx->GetValueOut();
        nFee = nDebit - nValueOut;
    }
    LOCK(pwallet->cs_wallet);
    
    for (unsigned int i = 0; i < tx->vout.size(); ++i)
    {
        const CTxOut& txout = tx->vout[i];
        isminetype fIsMine = pwallet->IsMine(txout);
        
        
        
        if (nDebit > 0)
        {
            
            if (pwallet->IsChange(txout))