Update dequeueOutputBuffer method

Add @Nullable annotation in the LibvpxVideoRenderer.

PiperOrigin-RevId: 263150736

extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/LibvpxVideoRenderer.java

@@ -239,6 +239,7 @@ public class LibvpxVideoRenderer extends SimpleDecoderVideoRenderer {
   }
 
   @Override
+  @Nullable
   protected VideoDecoderOutputBuffer dequeueOutputBuffer() throws VpxDecoderException {
     outputBuffer = decoder.dequeueOutputBuffer();
     return outputBuffer;