"Selfish mining is a deceitful cryptocurrency mining strategy in which one miner or a group solves a hash, opens a new block, and withholds it from the public blockchain. This action creates a fork, which is then mined to get ahead of the public blockchain."
It's not about mining empty blocks (why would you call not taking any fees selfish?).
The original 2018 paper on selfish mining is "Majority is not enough: bitcoin mining is vulnerable" [2] by Eyal and Gün Sirer, whose Abstract reads:
"The Bitcoin cryptocurrency records its transactions in a public log called the blockchain. Its security rests critically on the distributed protocol that maintains the blockchain, run by participants called miners. Conventional wisdom asserts that the mining protocol is incentive-compatible and secure against colluding minority groups, that is, it incentivizes miners to follow the protocol as prescribed.
We show that the Bitcoin mining protocol is not incentive-compatible. We present an attack with which colluding miners' revenue is larger than their fair share. The attack can have significant consequences for Bitcoin: Rational miners will prefer to join the attackers, and the colluding group will increase in size until it becomes a majority. At this point, the Bitcoin system ceases to be a decentralized currency.
Unless certain assumptions are made, selfish mining may be feasible for any coalition size of colluding miners. We propose a practical modification to the Bitcoin protocol that protects Bitcoin in the general case. It prohibits selfish mining by a coalition that command less than 1/4 of the resources. This threshold is lower than the wrongly assumed 1/2 bound, but better than the current reality where a coalition of any size can compromise the system."
More recent papers on selfish mining include"Effective Selfish Mining Defense Strategies to Improve Bitcoin Dependability" [3] and "Rethinking selfish mining under pooled mining" [4].
[1] https://www.investopedia.com/terms/s/selfish-mining.asp
[2] https://dl.acm.org/doi/10.1145/3212998
[3] https://www.mdpi.com/2076-3417/13/1/422
[4] https://www.sciencedirect.com/science/article/pii/S2405959522000443#b4