Don't show splash when thumbnail is null in SplashAlphaUseCaseTest

- thumbnail is always null to begin with, and we don't want the splash icon to ever shown in Overview, so it should default to hidden

Fix: 356079562
Test: SplashAlphaUseCaseTestTest
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Change-Id: I2a010f911814fda2bbcf0ca7ebc7192580772076