Allow empty adapter

Make wrapping adapter empty when the wrapped adapter is empty. Before when the wrapped adapter returned 0 from getCount(), signaling that it doesn't want to create any pages, the wrapping adapter returned 2. Because of that view pager could still call instantiateItem(). In my case my wrapped adapter wasn't ready at all to run instantiateItem() while getCount() returned 0 and it crashed. Thanks to this change view pager is blank until I add some data, which exactly what I expect.