Fix NPE when checking pending transition type

- This is a regression from ag/25618079, that change only affects the
  hide transition for an existing taskview so adding this check does
  not affect that behavior.

Bug: 317330143
Test: atest WMShellUnitTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:884f3c384b9936c49ebf6c69644a67f9ae8e5c44)
Merged-In: If1146c6023329b40b22fe78e0f7521534f9f62f3
Change-Id: If1146c6023329b40b22fe78e0f7521534f9f62f3