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).
if (bnb_used) {
                            coin_selection_params.use_bnb = false;
                            continue;
                        }
                        else {
                            error = _("Insufficient funds");
                            return false;
                        }
                    }
                } else {
                    bnb_used = false;
                }
                const CAmount nChange = nValueIn - nValueToSelect;
                if (nChange > 0)
                {
                    
                    CTxOut newTxOut(nChange, scriptChange);
                    
                    
                    
                    if (IsDust(newTxOut, discard_rate) || bnb_used)
                    {
                        nChangePosInOut = -1;
                        nFeeRet += nChange;
                    }
                    else
                    {
                        if (nChangePosInOut == -1)
                        {