SystemUI: Do not use null value animators

This commit ensures that any null value animators that are
passed out from the transition generator are handled properly.

Relates to 'Make sure AnimatorSet avoids null animations'
(change id Icfd751b1abceccf7f1177680b86d30610471810e)

Change-Id: I04357e33cfe293c2a1329a7033a5acb53bfb5c85