Address review.

extractor/src/main/java/org/schabi/newpipe/extractor/stream/AudioStream.java

@@ -415,6 +415,7 @@ public final class AudioStream extends Stream {
      *
      * @return the id of the audio track
      */
+    @Nullable
     public String getAudioTrackId() {
         return audioTrackId;
     }
@@ -424,6 +425,7 @@ public final class AudioStream extends Stream {
      *
      * @return the name of the audio track
      */
+    @Nullable
     public String getAudioTrackName() {
         return audioTrackName;
     }

extractor/src/test/java/org/schabi/newpipe/extractor/services/youtube/stream/YoutubeStreamExtractorDefaultTest.java

@@ -494,6 +494,12 @@ public class YoutubeStreamExtractorDefaultTest {
                             .stream()
                             .anyMatch(audioStream -> audioStream.getAudioTrackName().equals("English"))
             );
+
+            assertTrue(
+                    extractor.getAudioStreams()
+                            .stream()
+                            .anyMatch(audioStream -> audioStream.getAudioTrackName().equals("Hindi"))
+            );
         }
 
     }