Null check for fade start alpha value.

Bug 27105186
Fade previously hadn't had any values saved over those captured
by Visibility. If a Transition merged two Visibility transitions,
it could avoid calling captureStartValues for the Fade transition.
This change prevents an NPE in that case. The combined transition
won't get the intermediate alpha value, but at least it won't
crash.

Change-Id: I8e5720caafda56b017dfe1cc0b16ebdf246e90c4