Work around for NPE with shell transition

From the stack trace in the bug, the ActivityInfo for some reason is
null when it's passed to PipAppIconOverlay, work around the issue by
checking the variable to unblock the tests while we investigating the
underlying issue.

Bug: 272509531
Test: atest CtsAccessibilityServiceTestCases
Change-Id: I1536a2159aee47afe760908de3fb90c5882feff7