Fix null pointer exception on bubbles test

When no bubbles exist the following error occurs:

Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
07-28 04:11:01.355  1328  1368 E FLICKER : 	at com.android.wm.shell.flicker.bubble.MultiBubblesScreen$transition$1$2.invoke(MultiBubblesScreen.kt:79)

Bug: 240639586
Test: atest WMShellFlickerTests
Change-Id: I3ef42b65747aff5a4b4da7fa737efabbaba804fc