Fix NPE in PipTaskListener

There is a regression from ag/31449935 that we failed to do null-check
on the incoming PictureInPictureParams via onTaskInfoChanged.

Flag: com.android.wm.shell.enable_pip2
Bug: 393250416
Test: atest --iteration 5 \
      UsageStatsTest#testPipActivity \
      ActivityLifecycleKeyguardTests#testKeyguardShowHideOverPip \
      PinnedStackTests#testPipUnPipOverHome
Test: atest WMShellUnitTests:PipTaskListenerTest
Change-Id: I246b0615dfd9e9f5d2a583ca744f644351940d5e