chore: add kotlin sample codes (#404)

Author: jaeopt

android-sdk/src/main/java/com/optimizely/ab/android/sdk/OptimizelyClient.java

@@ -802,6 +802,7 @@ public OptimizelyUserContext createUserContext(@NonNull String userId,
         }
     }
 
+    @Nullable
     public OptimizelyUserContext createUserContext(@NonNull String userId) {
         return createUserContext(userId, null);
     }