Set language to null unless explicitly defined as "und" in container

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138758504

library/src/androidTest/assets/ogg/bear.opus.0.dump

@@ -22,7 +22,7 @@ track 0:
     encoderPadding = -1
     encoderPadding = -1
     subsampleOffsetUs = 9223372036854775807
     subsampleOffsetUs = 9223372036854775807
     selectionFlags = 0
     selectionFlags = 0
-    language = und
+    language = null
     drmInitData = -
     drmInitData = -
     initializationData:
     initializationData:
       data = length 19, hash BFE794DB
       data = length 19, hash BFE794DB

library/src/androidTest/assets/ogg/bear.opus.1.dump

@@ -22,7 +22,7 @@ track 0:
     encoderPadding = -1
     encoderPadding = -1
     subsampleOffsetUs = 9223372036854775807
     subsampleOffsetUs = 9223372036854775807
     selectionFlags = 0
     selectionFlags = 0
-    language = und
+    language = null
     drmInitData = -
     drmInitData = -
     initializationData:
     initializationData:
       data = length 19, hash BFE794DB
       data = length 19, hash BFE794DB

library/src/androidTest/assets/ogg/bear.opus.2.dump

@@ -22,7 +22,7 @@ track 0:
     encoderPadding = -1
     encoderPadding = -1
     subsampleOffsetUs = 9223372036854775807
     subsampleOffsetUs = 9223372036854775807
     selectionFlags = 0
     selectionFlags = 0
-    language = und
+    language = null
     drmInitData = -
     drmInitData = -
     initializationData:
     initializationData:
       data = length 19, hash BFE794DB
       data = length 19, hash BFE794DB

library/src/androidTest/assets/ogg/bear.opus.3.dump

@@ -22,7 +22,7 @@ track 0:
     encoderPadding = -1
     encoderPadding = -1
     subsampleOffsetUs = 9223372036854775807
     subsampleOffsetUs = 9223372036854775807
     selectionFlags = 0
     selectionFlags = 0
-    language = und
+    language = null
     drmInitData = -
     drmInitData = -
     initializationData:
     initializationData:
       data = length 19, hash BFE794DB
       data = length 19, hash BFE794DB

library/src/androidTest/assets/ogg/bear.opus.unklen.dump

@@ -22,7 +22,7 @@ track 0:
     encoderPadding = -1
     encoderPadding = -1
     subsampleOffsetUs = 9223372036854775807
     subsampleOffsetUs = 9223372036854775807
     selectionFlags = 0
     selectionFlags = 0
-    language = und
+    language = null
     drmInitData = -
     drmInitData = -
     initializationData:
     initializationData:
       data = length 19, hash BFE794DB
       data = length 19, hash BFE794DB

library/src/main/java/com/google/android/exoplayer2/extractor/ogg/OpusReader.java

@@ -81,7 +81,7 @@ import java.util.List;
 
 
       setupData.format = Format.createAudioSampleFormat(null, MimeTypes.AUDIO_OPUS, null,
       setupData.format = Format.createAudioSampleFormat(null, MimeTypes.AUDIO_OPUS, null,
           Format.NO_VALUE, Format.NO_VALUE, channelCount, SAMPLE_RATE, initializationData, null, 0,
           Format.NO_VALUE, Format.NO_VALUE, channelCount, SAMPLE_RATE, initializationData, null, 0,
-          "und");
+          null);
       headerRead = true;
       headerRead = true;
     } else {
     } else {
       boolean headerPacket = packet.readInt() == OPUS_CODE;
       boolean headerPacket = packet.readInt() == OPUS_CODE;