Handle the case where transition may have empty immersive change

During an exit, the transition may end up not containing the immersive
change if it was picked up by an earlier transition, such as when
entering PIP. In those case, do not require that the immersive task be
present or a crash may occur.

Fix: 377607197
Flag: com.android.window.flags.enable_fully_immersive_in_desktop
Test: enter/exit fullscreen immersive with Neftlix multiple times
Change-Id: I0a60bd25484f44f75d54a3b7ab789596511cd890