Accept setting null override to setSelectionOverride

In-code documentation say this is allowed, and is the expected way to
disable renderer.

library/core/src/main/java/com/google/android/exoplayer2/trackselection/DefaultTrackSelector.java

@@ -646,7 +646,7 @@ public class DefaultTrackSelector extends MappingTrackSelector {
      * @return This builder.
      */
     public final ParametersBuilder setSelectionOverride(
-        int rendererIndex, TrackGroupArray groups, SelectionOverride override) {
+        int rendererIndex, TrackGroupArray groups, @Nullable SelectionOverride override) {
       Map<TrackGroupArray, SelectionOverride> overrides = selectionOverrides.get(rendererIndex);
       if (overrides == null) {
         overrides = new HashMap<>();