Remove non-emptyness assertion for insets

Some devices might not have a navigation bar and some SplashScreenTests
might fail since we were asserting for the non-emptynesss of the insets.

Test: set config_showNavigationBar=false in config.xml files and run the
test
Bug: 195732616

Change-Id: I9cf2595bb8017ab9d07a7e2180d9eb18bca70200