[sf] Release the currently presented buffer when setBuffer is called

with null

Fixes a regression introduced in T which ignores a setBuffer call
with a null buffer. The expected behavior should be to release the
currently presented buffer from surfaceflinger. The subsequent frame
will not present this layer so the region behind the layer will be
composited instead.

Bug: 241271897
Test: presubmit
Change-Id: Ie06025c59c58cc75a267b783729996a3cbceef45