大乘起信論義記 2.57

2.57

言如實空者，此以如實之中，空無妄染故，云如實空，非謂如實自空。此則如實之空，依主釋也。以妄空故，遂能顯示真理，故云顯實也。故《中邊論》云「無能取所取有，有能取所取無，是名空相」故也。

Regarding "empty in accordance with reality" (tathā-śūnya): this means that within reality, it is empty of delusional defilements, hence called "empty in accordance with reality"; it does not mean that reality is itself empty. This is "emptiness of reality," a tatpuruṣa compound. Because delusion is empty, it can thereby reveal the true principle; hence it is said to "reveal the real." Therefore the Madhyāntavibhāga says: "The existence of the non-existence of grasper and grasped, and the non-existence of the grasper and grasped, is called the mark of emptiness."