[flexiglass] Update expected empty shade message in test.

The test for `EmptyShadeViewModel` now checks for "You're all caught up" without an exclamation mark.

Fix: 441276974
Test: Unit test now passes.
Flag: com.android.systemui.scene_container
Change-Id: I73717008e6c6c7be91b747219195cefb7afda0d9