[nullness] The nullness checking fixes of the media related files

We rolled back the previous <unknown commit> because some of the nullness checking fixes broke the exoplayer2 tests. We submit this CL into the TAP Global Presubmit train (https://test.corp.google.com/OCL:337620582:BASE:338201100:1603283151742:36afd5a5) and make sure that this CL wouldn't break any other tests on google3.

PiperOrigin-RevId: 338267548

extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultMediaItemConverter.java

@@ -45,7 +45,9 @@ public final class DefaultMediaItemConverter implements MediaItemConverter {
   @Override
   @Override
   public MediaItem toMediaItem(MediaQueueItem item) {
   public MediaItem toMediaItem(MediaQueueItem item) {
     // `item` came from `toMediaQueueItem()` so the custom JSON data must be set.
     // `item` came from `toMediaQueueItem()` so the custom JSON data must be set.
-    return getMediaItem(Assertions.checkNotNull(item.getMedia().getCustomData()));
+    MediaInfo mediaInfo = item.getMedia();
+    Assertions.checkNotNull(mediaInfo);
+    return getMediaItem(Assertions.checkNotNull(mediaInfo.getCustomData()));
   }
   }
 
 
   @Override
   @Override