Make sure AnimatorSet avoids null animations

This commit makes the animator set avoid null value animation objects
as much as possible. No comments about these have been expressed in
the documentation so they are just treated as being animations that
happen instantly and have no real effect meaning they are just
ignored. The null value checks are enforced in both the builder and
the extra helper commands allowing all the methods to receive null
values.

Change-Id: Icfd751b1abceccf7f1177680b86d30610471810e