code: public static <T> SortedSet<T> nullToEmpty(final SortedSet<T> set) {
          return set == null ? N.<T> emptySortedSet() : set;
      }
docs: Returns an immutable empty <code>SortedSet</code> if the specified SortedSet is <code>null</code>, otherwise itself is returned.
  
  @param set
  @return