Fix null pointer exception when getting app label for headless app

Will follow up with regression test in CrossProfileAppsTest cts
test.

Test: Manual - go to settings -> Digital WellBeing -> Confirm switch profile button is shown
Test: atest frameworks/base/core/tests/coretests/src/android/content/pm/CrossProfileAppsTest.java
Bug: 295888390
Change-Id: I2c95a73434fa724f69a945809106e4de3b59a6ba