Fix TransitionManager.go() to allow null transitions

The doc'd behavior of go() is that a null transition is equivalent
to just switching to a scene with no transition. The actual behavior
was an NPE crash.

Issue #17342291 TransitionManager.go() with null Transition parameter throws NPE unlike documentation

Change-Id: I9416aa6e307f80eefc201ca3264fd64f8587ac2f