Prevent Cea608Decoder from generating Subtitles with null Cues list.

PiperOrigin-RevId: 223580953

library/core/src/main/java/com/google/android/exoplayer2/text/cea/Cea608Decoder.java

@@ -33,6 +33,7 @@ import com.google.android.exoplayer2.text.SubtitleInputBuffer;
 import com.google.android.exoplayer2.util.MimeTypes;
 import com.google.android.exoplayer2.util.ParsableByteArray;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -451,7 +452,7 @@ public final class Cea608Decoder extends CeaDecoder {
 
     switch (cc2) {
       case CTRL_ERASE_DISPLAYED_MEMORY:
-        cues = null;
+        cues = Collections.emptyList();
         if (captionMode == CC_MODE_ROLL_UP || captionMode == CC_MODE_PAINT_ON) {
           resetCueBuilders();
         }
@@ -506,7 +507,7 @@ public final class Cea608Decoder extends CeaDecoder {
     if (oldCaptionMode == CC_MODE_PAINT_ON || captionMode == CC_MODE_ROLL_UP
         || captionMode == CC_MODE_UNKNOWN) {
       // When switching from paint-on or to roll-up or unknown, we also need to clear the caption.
-      cues = null;
+      cues = Collections.emptyList();
     }
   }