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).
{
               for (const CTxOut& txout : wtx.tx->vout)
                   if (IsChange(txout))
                   {
                       CTxDestination txoutAddr;
                       if(!ExtractDestination(txout.scriptPubKey, txoutAddr))
                           continue;
                       grouping.insert(txoutAddr);
                   }
            }
            if (grouping.size() > 0)
            {
                groupings.insert(grouping);
                grouping.clear();
            }
        }
        
        for (const auto& txout : wtx.tx->vout)
            if (IsMine(txout))
            {
                CTxDestination address;
                if(!ExtractDestination(txout.scriptPubKey, address))
                    continue;
                grouping.insert(address);
                groupings.insert(grouping);
                grouping.clear();
            }
    }
    std::set< std::set<CTxDestination>* > uniqueGroupings; 
    std::map< CTxDestination, std::set<CTxDestination>* > setmap;