Check for emtpy list instead of list.size =0

Checking for an empty list will give better error messages when the test
fails.

Cherry picked from http://ag/20455368

Bug: 257245061
Test: atest CtsMediaHostTestCases:android.media.metrics.cts.MediaMetricsAtomTests#testAppBlocklis
Change-Id: I1c190841fe00dd84a2369342f9abaa4c15caf22d
Merged-In: I1c190841fe00dd84a2369342f9abaa4c15caf22d