tax rates do not cache if empty

If no tax rates are set this query happens for every contribution that is imported (including via api) as the caching does not work

/**

• Get all tax rates.
  *
• The static array tax rates is returned
  *
• @return array
• array list of tax rates with the financial type
  */
  public static function getTaxRates() {
  if (!self::$taxRates)
  Unknown macro: { self}

return self::$taxRates;
}