DesktopMixed: check we have no valid changes if launch change is null

Sometimes a pending launch transition turns into a different type of
transitionthat DesktopMixedTransitionHandler doesn't handle. In this CL
verify that we are not expecting to make any minimize or immersive-exit
changes in such transitions.

Test: manual
Bug: 327428659
Flag: com.android.window.flags.enable_desktop_app_launch_transitions
Change-Id: I110a9079af9d459a97b2d20d30450700bf3cf78f