Fix issue where a fling followed quickly by a null dropped the dismissFling.

This reproduced sporadically in tests and caused swiping to set up the test to fail.

Leaving detailed logs in case this does not fix the issue, but will remove once the fix is verified.

Fixes: 417163416
Test: could not reproduce locally/with abtd 99x retry, but verified with logs that this is the issue
Flag: com.android.systemui.keyguard_wm_state_refactor
Change-Id: Ib70af4fb358058ff158fb223badc1ff94eaacd46