Fix validation crash for nextPageToken.

We are caching nextPageToken in query(), and validate it in
getNextPage(). But when it reach to the end, the token will be changed
to 0. And it will crash if user is trying to fetch next page since 0
is not cached and won't passed the validation.

This change fix the issue. Without it, users maybe crashed if they
are trying to retrieve all documents in all result pages.

Bug: 187972715
Test: AppSearchSessionUnitTest
Change-Id: Ia95feb52f2a37348e11afdf0b320c61bfce22d40