Null check before writing RemoteAnimationTarget proto

Change-related variables are null when not changing, thus
we need to check for that before trying to write them to proto

Bug: 123538318
Test: ActivityLifecycleTopResumedStateTests#testTopPositionLostOnFinish
      passes

Change-Id: I643c48621602b5203f7f64a9af0578273f115dbe