Step clock animation: Fix NPE when startValues or endValues are null

By looking at parent class implementations of Transition, it seems that
is a legit case, and that should be handled by returning a null Animator

Test: SplitShadeTransitionAdapterTest.kt
Fixes: 274002399
Change-Id: I07501eb4bbd001bdb6014a3644e41cd8b575ccee
2 files changed