code: @Nullable
    public T selectTemplate(
        String delTemplateName, String variant, Predicate<String> activeDelPackageSelector) {
      Group<T> group = nameAndVariantToGroup.get(delTemplateName, variant);
      if (group != null) {
        T selection = group.select(activeDelPackageSelector);
        if (selection != null) {
          return selection;
        }
      }
      if (!variant.isEmpty()) {
        // Retry with an empty variant
        group = nameAndVariantToGroup.get(delTemplateName, "");
        if (group != null) {
          return group.select(activeDelPackageSelector);
        }
      }
      return null;
    }
docs: Returns an active delegate for the given name, variant and active packages. If no active
  delegate if found for the {@code variant} the we fallback to a non variant lookup. Finally, we
  return {@code null} if no such template can be found.
  
  <p>See {@code soy.$$getDelegateFn} for the {@code JS} version