Add clarification to TextOutput doc.

The list may be empty if no cues are available.

PiperOrigin-RevId: 223527105

library/core/src/main/java/com/google/android/exoplayer2/text/TextOutput.java

@@ -25,8 +25,7 @@ public interface TextOutput {
   /**
    * Called when there is a change in the {@link Cue}s.
    *
-   * @param cues The {@link Cue}s.
+   * @param cues The {@link Cue}s. May be empty.
    */
   void onCues(List<Cue> cues);
-
 }