Call back with null AssistContent if request fails

Ensures that the onAssistContent method is always called (with null if
necessary).

Bug: 341017571
Fix: 341017571
Test: manual via checking that the callback in the
ScreenshotActionsProvider is called
Flag: com.android.systemui.screenshot_shelf_ui2

Change-Id: I762e6fe96c24f4b60fcb15285c38873faf166d91