handle empty NAL when looking for HEVC layerId

parseHEVCLayerId() would CHECK() if there was no data, we now return
that there was no valid HEVC layer info. This was triggered by changes
so a bad NAL would be skipped rather than aborting.

Bug: 127833120
Bug: 122749422
Test: re-ran CTS
Change-Id: I239697f9d1f77995f9ccf5e8c77cfaa01994e326