private static <T> Collection<T> nullIfEmpty(Collection<T> source) {
        return source == null || source.isEmpty() ? null : source;
    }
private static < T > Collection < T > nullIfEmpty ( Collection < T > source ) { return source == null || source . isEmpty ( ) ? null : source ; }
@param source can be <code>null</code>.
@param <T>    the collection type.
@return the collection, or <code>null</code> if the collection
id <code>null</code> or empty.