Prevent text tracks with no language being selected by default

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154421706

library/core/src/main/java/com/google/android/exoplayer2/trackselection/DefaultTrackSelector.java

@@ -867,7 +867,8 @@ public class DefaultTrackSelector extends MappingTrackSelector {
   }
 
   protected static boolean formatHasLanguage(Format format, String language) {
-    return TextUtils.equals(language, Util.normalizeLanguageCode(format.language));
+    return language != null
+        && TextUtils.equals(language, Util.normalizeLanguageCode(format.language));
   }
 
   // Viewport size util methods.