Fix NPE in SwipeAnimation

This CL fixes a crash that could happen when a user action is invalid.
In some cases, we would still start the transition but the content used
to compute the swipe distance would not be composed, leading to a crash.

Bug: 416260000
Test: SwipeToSceneTest
Flag: EXEMPT bug fix
Change-Id: I00dfb04e414819106fc0841490f355598d7412b1