Add example of making some params nullable.
Add example of making some params nullable.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112487033

google/auto,MaTriXy/auto,MaTriXy/auto,eamonnmcmanus/auto,eamonnmcmanus/auto,eamonnmcmanus/auto,google/auto,google/auto