Log.wtf if display list is empty.

This should never happen and is a significant issue if it does, as
it can prematurely end the screenshot pipeline.

Bug: 360843936
Test: Manually tested normal screenshot usage to ensure it doesn't
      trigger this case.
Flag: EXEMPT Adding log
Change-Id: I544cee1e09e4dccbd2c8bbd76eda0f4f9e63395b