PageRequest should support having a null next key docId

PageRequest should support having a null next key docId
the Page Link JSON String is now generated in a way that supports a null docId even when the key is not null. This bug made asJson() method to fail when docId was null. Also, as toString() was invoking asJson(), this made the debugging harder, so the toString() method should not invoke asJson() but rather basically print the Object's properties.