Fix NPE when passing null TvContentRating

Change-Id: I80b0ca60b1623356fd39015521b50ae9f1eaef07
diff --git a/media/java/android/media/tv/TvInputManager.java b/media/java/android/media/tv/TvInputManager.java
index f76c78b..815ba52 100644
--- a/media/java/android/media/tv/TvInputManager.java
+++ b/media/java/android/media/tv/TvInputManager.java
@@ -1339,6 +1339,9 @@
                 Log.w(TAG, "The session has been already released");
                 return;
             }
+            if (unblockedRating == null) {
+                throw new IllegalArgumentException("unblockedRating cannot be null");
+            }
             try {
                 mService.requestUnblockContent(mToken, unblockedRating.flattenToString(), mUserId);
             } catch (RemoteException e) {