Fixes NullPointerException

When the Launcher Task has an empty child TaskFragment, the Launcher
Task is no longer a leaf TF. In that case, starting a home activity
of the secondary user will create another Launcher Task/Activity and
NPE happens while trying to pause the existing Launcher.

Bug: 331525785
Test: wm presubmit
Change-Id: I9f6116ac1f6c8f5bb2094f9adb539177758e684e