SDK / exoplayer

When the first streaming license request response provided to mediaDrm it might return an empty array instead of null.
This was set to offlineLicenseKeySetId which made the work like there is a valid offline license.
Simplified the code and made it to set offlineLicenseKeySetId only if there is sensible data in keySetId.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144843144