fix(graphql): nextPage and prevPage are non nullable even though they can be null sometimes

[r1tsuu]

This PR introduced #11952 improvement for graphql schema with making fields of the Paginated<T> interface non-nullable.

However, there are a few special ones - nextPage and prevPage. They can be null when:
The result returned 0 docs.
The result returned x docs, but in the DB we don't have x+1 doc. Thus, nextPage will be null. The result will have nextPage: null. Finally, when we query 1st page, prevPage is null as well.