PagedProductQuery constructor Null safety

1. smooth_app
2. paged_product_query
3. PagedProductQuery
4. PagedProductQuery constructor

PagedProductQuery class

1. Constructors
2. PagedProductQuery
3. Properties
4. hashCode
5. pageNumber
6. pageSize
7. runtimeType
8. Methods
9. getParameter
10. getSearchResult
11. noSuchMethod
12. toNextPage
13. toString
14. toTopPage
15. Operators
16. operator ==