Make MediaSource.Factory a nullable parameter.

In some cases, callers will need to pass in both BitmapLoader and Codec.DecoderFactory without specifying a custom MediaSource.Factory. Omitting the annotation will result in NULL_FOR_NONNULL_TYPE compilation errors in Kotlin.

PiperOrigin-RevId: 695481606

libraries/transformer/src/main/java/androidx/media3/transformer/DefaultAssetLoaderFactory.java

@@ -125,7 +125,7 @@ public DefaultAssetLoaderFactory(
       Context context,
       Codec.DecoderFactory decoderFactory,
       Clock clock,
-      MediaSource.Factory mediaSourceFactory,
+      @Nullable MediaSource.Factory mediaSourceFactory,
       BitmapLoader bitmapLoader) {
     this.context = context.getApplicationContext();
     this.decoderFactory = decoderFactory;