An option contained in any lease to purchase the whole or any part of the leased premises, exercisable either during the term of the lease or immediately upon its termination, shall, if otherwise valid, be enforceable in accordance with its terms, irrespective of the rule against perpetuities. In any suit to enforce such option, the lessor, or the successors in interest of the lessor, shall not plead the rule against perpetuities as a defense, and the same shall not constitute a defense either in law or in equity: Provided, That this section shall not apply to any lease heretofore executed.