Fix REST API for popstore.ProductServices.get#SortedProductList

[acetousk]

As per: https://forum.moqui.org/t/is-there-a-api-in-popreststore-which-can-get-all-the-products/155/3

[jonesde]

What is the scenario this is meant to accommodate? Doing a query on ProductCategoryMemberProduct with no productCategoryId would in effect find all products that are in any category. It might be best if this treated the no category scenario the same as no products in the category (ie if a category is not specified then no products found).

On a side note, the same effect as this patch can be done with the ignore-if-empty="true" attribute on the econdition element, but either way I don't think this is the business logic we want here.