Moving check for empty snapshots earlier.

- This also prevents them from being added to the cache and reporting that
  the snapshot changed.

Bug: 62727429
Test: bit FrameworksServicesTests:com.android.server.wm.TaskSnapshotControllerTest,com.android.server.wm.TaskSnapshotPersisterLoaderTest,com.android.server.wm.TaskSnapshotCacheTest

Change-Id: Ie02f120e5460322da28faa8a5679191024377c4c
Signed-off-by: Winson Chung <winsonc@google.com>