It’s a good question, and I think it may have been discussed in the literature somewhere, but I can’t recall where off the top of my head.
Luckily, SuttaCentral is incredibly awesome and I just found a couple of Sanskrit texts that include this passage. They are in Chapters Mvu 73 and Mvu 110 of the Mahāvastu, a Vinaya compilation of the Mahasanghika-Lokuttaravadin school.
This text is clearly a late compilation, which includes a lot of legendary material. But it is a composite, with all kinds of things plunked in there, with little regard for consistency or style. This is terrible for readers but great for textual scholars, as it preserves things from different sources, apparently without much alteration.
The phrase in Mvu 73 is:
śūnyā ātmena vā ātmanīyena vā
Empty of self and of what belongs to self.
In Mvu 110 it is:
śūnyā anātmanīyā ātmena vā ātmanīyena vā
Empty and not belonging to self of a self and what belongs to a self.
See what I mean? Mvu 110 appears to have an extra term added, rather clumsily.
Anyway, clearly here “emptiness” is associated with not-self, and it is the same general phrase as found in the Theravada and Sarvastivada texts. The context is different, though, and so far as I can see at a glance, what is happening is that there’s a discussion of not-self, and this phrase is referred to in the course of the discussion, as if it was a well-known term. So it’s not the same sutta, but it is the same term, and appears to be part of a body of well-known phrases.
Now, the Mahasanghika and the Sthavira group of schools were the first to split, or at least, the first to formalize a split, so it is generally assumed that common material dates to the pre-sectarian times. There seems no reason to doubt this here, and so it would seem as if the distinctly different Sarvastivadin phrasing may be later.
Without closer study, however, it’s hard to say if this phrase was a feature of the Sanskrit original or an extra explanation supplied by the Chinese translator. My money would be on the former, but it’s just a guess.