what's the emptiness, 空, in 3rd jhana of STED 4 jhana formula in agamas?

The 4 jhanas standard formula in the agamas of MA, match up very closely with the pali STED 4 jhana formulas. There's a striking difference in the 3rd jhana though, which is not in the pali. There's a sunyata/kong/空 on this part of third jhana:

聖所捨、念、 樂住、 shèng suǒ shě, niàn, lè zhù	the noble’s equanimity (捨), mindfulness, happy (樂) abode,
空，kōng	and emptiness.
得第三禪成就遊， dé dì sān chán chéngjiù yóu	He attains the accomplishment of the third meditation.

what is that emptiness?

Mystery solved, reading another translation of MA 2 with footnote

THE MADHYAMA ĀGAMA
MIDDLE-LENGTH DISCOURSES)
VOLUME I

footnote 47:
 Le zhu shi, Pāli sukha-vihāra. In the Chinese textual tradition the final character shi,
“room, abode,” is sometimes mistaken as kong, “emptiness.” Another variant is ding,
“concentration.”

That kong/空 is a scribal error is supported by the lack of a corresponding
term in the Pāli version of the formula.

full context

from MA 2, madhyama agama

(first jhāna)

復次，Fù cì	“Furthermore,
聖弟子離欲、 shèng dìzǐ lí yù,	the noble disciple parts with desire (欲 kāmehi)
離惡不 善之法， lí è bùshàn zhī fǎ	and parts with evil (惡) or un-wholesome (不-善) Dharmas (之法).
有覺、有觀，yǒu jué, yǒu guān,	With thought (覺 vitakka) and investigation (觀 vicāra),
離生喜樂， lí shēng xǐlè,	this seclusion produces joy (pīti 喜) and happiness (sukha 樂),
得初禪成就 遊，dé chū chán chéngjiù yóu,	and he attains the accomplishment of the first meditation (禪 zen, jhāna).
是時，聖弟子名葉還生，猶三十三天晝度 樹葉還生也。	The noble disciple then is called ‘leaves growing back again’ like the leaves growing back on the Trāyastriṃśa Heaven’s Pārijāta Tree.

(second jhana)

復次，聖弟子 Fù cì, shèng dìzǐ	“Furthermore, once the noble disciple’s
覺觀已息，jué guān yǐ xī	thought and investigation have stopped,
內靜、 nèi jìng	he possesses an inner (內 adjhattam) stillness (靜 sampasadanam)
一心，yīxīn	(and) unified (ekodi-bhava) mind (心 cetaso)
無覺無觀， wú jué wú guān	without (無) thought (覺 vitakka) or investigation (觀 vicara).
定生喜樂，dìng shēng xǐlè	This concentration (定 samadhi) produces (生) joy (喜 piti) and happiness (樂 sukha),
得第二禪成就 遊，dé dì èr chán chéngjiù yóu	and he attains the accomplishment of the second meditation (禪 jhana, zen).
是時，聖弟子名為生網，猶三十三天晝度 樹生網也。	The noble disciple then is called ‘growing webs of buds’ like the Trāyastriṃśa Heaven’s Pārijāta Tree growing webs of buds.

(third jhana)

復次，聖弟子 Fù cì, shèng dìzǐ	“Furthermore, the noble disciple
離於喜欲，lí yú xǐ yù	parts with joy and desire,
捨無求 遊，shě wú qiú yóu	and he is equanimous (捨 upekkha) without further pursuit.
正念正智 zhèngniàn zhèngzhì	With right mindfulness and right knowledge,
而身覺樂，ér shēn jué lè	he experiences (覺 patisamvedi) happiness (樂 sukha) with the body (身)
謂聖所說、 wèi shèng suǒ shuō	that’s described by the nobles as
聖所捨、念、 樂住、 shèng suǒ shě, niàn, lè zhù	the noble’s equanimity (捨), mindfulness, happy (樂) abode,
空，kōng	and emptiness.
得第三禪成就遊， dé dì sān chán chéngjiù yóu	He attains the accomplishment of the third meditation.
是時，聖弟子名 生如鳥喙，猶三十三天晝度樹如鳥喙也。	The noble disciple then is called ‘growing buds like bird beaks’ like the Trāyastriṃśa Heaven’s Pārijāta Tree growing buds like bird beaks.

(fourth jhana)

復 次，聖弟子 Fù cì, shèng dìzǐ	“Furthermore, the noble disciple’s
樂滅、苦滅，lè miè, kǔ miè	pleasure (sukha) ceases and his pain (dukkha) ceases.
喜憂本已滅，xǐ yōu běn yǐ miè	The basis of joy (so-manassa) and sorrow (do-manassa) having ceased,
不苦不樂、 bù kǔ bù lè	he is neither (a-dukkham-a-sukham) discomforted nor delighted.
捨、念、清淨，shě, niàn, qīngjìng	Equanimous, mindful, and pure (pari-suddhim),
得第四禪成就遊，dé dì sì chán chéngjiù yóu	he attains the accomplishment of the fourth meditation.
是時，聖弟子 名生如鉢，猶三十三天晝度樹如鉢也。	The noble disciple then is called ‘becoming like a bowl’ like the Trāyastriṃśa Heaven’s Pārijāta Tree buds becoming like bowls.