[CTS] Assert non-null instead of throwing NPE

Add non-null assertion to avoid NullPointerException. This only improves
the error message. The test failure seems caused by transient infra
issue and is passing now.

Bug: 332766201
Test: atest ActivityEmbeddingCrossUidTests
Change-Id: I30b1cd11e8fb42e8c20b471c942725818bbf8be3