SF: fix BufferTX counter for null buffers

Don't increment the BufferTX counter is the transaction includes
a buffer change but that buffer is null.

Test: Open  https://jsfiddle.net/8q7nuwh9/show in Chrome and scroll up down
Bug: 192352790
Change-Id: Ia0a27ff32fb20f20a1ac671f2b4fdb3ae22a8aea