Prevent NPE when placing fading elements

This CL fixes an NPE that can happen since http://ag/27685078: the
placeWithLayer block can still run even if an element is removed from
composition.

Bug: 290930950
Test: Manual. Unfortunately it's impossible to add tests for this
 because the timings solely depend on the invalidation of the graphics
 layer by the Compose runtime. I have not observed the NPE with this
 change, but I also didn't have manual reproduction steps.
Flag: com.android.systemui.scene_container
Change-Id: Ic8bd91b3d6fd3f8b3d697c72a42bb5f278f5d207