Allow sink provider to be flushed before initialized

VideoSink.initialize will be added in a next CL and we want to allow
flush to be called before and after initialize.

PiperOrigin-RevId: 624946957

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/video/CompositingVideoSinkProvider.java

@@ -505,6 +505,9 @@ public final class CompositingVideoSinkProvider
   }
 
   private void flush() {
+    if (!isInitialized()) {
+      return;
+    }
     pendingFlushCount++;
     // Flush the render control now to ensure it has no data, eg calling isReady() must return false
     // and render() should not render any frames.