[Pagination] callback property default value is not specified.

In docs on pagination control, default value of page factory is not specified.
Now, I see, that it is null. I mean, If I call (new Pagination().getPageFactory()) == null.

But some time before, when I wrote tests, I used checking:
assertTrue((new Pagination().getPageFactory()) instanceof Callback);
which was working well. But now it fails, because null is not an instance of Callback. So test fails.

So I decided to request specifying this information in javadoc.

Also, usage of null as value of page factory property is not specified in javadoc.