Allow null child view when checking whether divider should draw

The beginning divider should draw before the first child that is not
set to GONE. Incidentally, there needs to be a check for such a child,
which involves checking the visibility of all the children with lower
indices. This CL takes into account the scenarios where children with
lower indices could be null, in which case we'll treat them as if they
were GONE.

Bug: 22199594
Change-Id: Iefc73ef87beec219898c967b9a3ccebd1e3b5b4c