Check for null buffer when calling Transaction#setBuffer

android_hardware_HardwareBuffer_getNativeHardwareBuffer will crash when
trying to convert a null object to a GraphicBuffer. Instead, just set
the GraphicBuffer to null and call into SCC with a null buffer. This
will result in the previous buffer getting released

Test: SurfaceControlTest
Fixes: 233252754
Change-Id: Idc9f5ca3d747dbc890d7cafb9512c51ca9e711c9