Prevent possibly buggy update of dsr for figures.

* It is not necessary that both dsr[0] and dsr[1] be present always.
  So, if a value is null, it should not be updated by the delta.

* Unclear if this will be a real source of bugs, but better safe
  than sorry.

* Also, updated comments.

Change-Id: I1b815c5ab7607a1f5fba57ab67f7f99358ea5378