divide by zero in OrderTotalCalculationService UpdateOrderTotals

1. nopCommerce version
3.80  (also checked in 4.00 and did not see any code preventing divide by zero in this case)

2. Expected behavior
be able to update product qty from admin backend.

3. Actual behavior
system crashings with divide by zero error

4. Steps to reproduce the problem
Make sure AutoUpdateOrderTotalsOnEditingOrder = true

Create an order and add 1 product (regular product that has a cost e.g 10$ item). Also add another product that does not have a cost (e.g 0$)

Once order is placed, if admin goes to edit qty on an order item on that order, the code will error out when trying to calculate the taxRate since its dividng by itemSubTotalExclTax which is zero.

5. Any private modifications you made to your nopCommerce

I overrode IOrderTotalCalculationService and just checked if itemSubTotalExclTax > 0 before dividing.

if (itemSubTotalExclTax > 0
      taxRate = Math.Round((100 * (itemSubTotalInclTax - itemSubTotalExclTax)) / itemSubTotalExclTax, 3);

Done. Please see these commit.
https://github.com/nopSolutions/nopCommerce/commit/afce7e1db94b332f0ab70f0a99e23977317853cb