Fix Hackbook crashing.
Fix Hackbook crashing.

Summary: Adding tokens to the TokenCache causes NPEs when the tokens are null. Check against that before adding the tokens.

Test Plan: ran Hackbook test app, verified that the app works now.

Reviewers: mmarucheck, francip, clang, karthiks

Reviewed By: francip

CC: msdkexp@

Differential Revision: https://phabricator.fb.com/D563332

Task ID: 1408738

Flutterbee/facebook-android-sdk,ribot/facebook-android-sdk,TechSmith/facebook-android-sdk,lizhi5753186/facebook-android-sdk,RabbleApp/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,minhduc3404/facebookSdk,TimeIncOSS/facebook-android-sdk,Limbika/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,RabbleApp/facebook-android-sdk,taisukeoe/facebook-android-sdk,scalio/facebook-android-sdk,PioneerLab/facebook-android-sdk,quizlet/facebook-android-sdk,couchbaselabs/facebook-android-sdk,bitcellar-labs/facebook-android-sdk,lizhi5753186/facebook-android-sdk,taisukeoe/facebook-android-sdk,mindjolt/facebook-android-sdk,Limbika/facebook-android-sdk,MKrishtop/facebook-android-sdk,TechSmith/facebook-android-sdk,couchbaselabs/facebook-android-sdk,MarkHappy/Facebook_SDK,PioneerLab/facebook-android-sdk,mindjolt/facebook-android-sdk,GrioSF/facebook-android-sdk,MKrishtop/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,MarkHappy/Facebook_SDK,Flutterbee/facebook-android-sdk,TimeIncOSS/facebook-android-sdk,bitcellar-labs/facebook-android-sdk,ribot/facebook-android-sdk,scalio/facebook-android-sdk,GrioSF/facebook-android-sdk,quizlet/facebook-android-sdk,minhduc3404/facebookSdk