如何防止表值函数参数的空值？

SELECT * FROM my_table WHERE column_name = COALESCE(NULLIF(param, ''), 'default_value');

SELECT * FROM my_table WHERE column_name = NULLIF(param, '');

SELECT * FROM my_table WHERE column_name = CASE WHEN param IS NULL OR param = '' THEN 'default_value' ELSE param END;

CREATE FUNCTION my_function(param VARCHAR(255))
RETURNS TABLE (column_name VARCHAR(255))
LANGUAGE plpgsql
AS $$
BEGIN
  IF param IS NULL OR param = '' THEN
    RETURN QUERY SELECT 'default_value'::VARCHAR(255);
  ELSE
    RETURN QUERY SELECT param::VARCHAR(255);
  END IF;
END;
$$;