首楞嚴義疏注經 10.122

10.122

陰既因果皆無，涅槃因果亦皆斷滅。斯則有為、無為，染、淨諸法因果俱無，故云一切皆空。

Since the aggregates are devoid of both cause and fruit, the cause and fruit of nirvāṇa are also both annihilated. Thus, conditioned and unconditioned, defiled and pure dharmas, are all devoid of cause and fruit; hence it says 'all are empty'.