Let CastPlayer#getTrackSelector return null

Change CastPlayer#getTrackSelector to return null
instead of throwing exception.

PiperOrigin-RevId: 310319964

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

@@ -517,7 +517,7 @@ public final class CastPlayer extends BasePlayer {
   @Override
   @Nullable
   public TrackSelector getTrackSelector() {
-    throw new UnsupportedOperationException();
+    return null;
   }
 
   @Override

library/core/src/main/java/com/google/android/exoplayer2/Player.java

@@ -1190,7 +1190,9 @@ public interface Player {
    */
   int getRendererType(int index);
 
-  /** Returns the track selector that this player uses. */
+  /**
+   * Returns the track selector that this player uses, or null if track selection is not supported.
+   */
   @Nullable
   TrackSelector getTrackSelector();