fix sort value resetting

[DevertNet]

If the attribute is not filled, no products will be found with this query. This will result in a wrong product order.
For performance reasons you could add a if condition in the foreach.

[punkstar]

@DevertNet Thanks for the contribution, but I don't follow what's going on here and what your change fixes.

As I understand it the collection that you modified is looking for all products that have a quantity that is not zero and setting it to zero. That means that all products in the database now have an integer there and not a null value. The aim of that code isn't to zero the entire catalogue.

Is there a specific bug that you're fixing?

[DevertNet]

Yes, if the products have a null value, the sorting will be wrong.
Depending on ASC/DESC the result will be -1, -2, -3, null, null, null, ... or null, null, null, ..., -3, -2, -1, .
If they have a zero (0) value it will be: -3, -2, -1, 0, 0, 0, ... or 0, 0, 0, ..., -1, -2, -3.

[sprankhub]

I think #4 fixes your issue @DevertNet.