Firstly you must understand that emptiness does not mean an nihilistic idea of non-existence or void.
But rather, emptiness refers to the truth that all things are conditioned arising, interdependent co-arising. Since all are conditioned arising, everything lacks inherent existence.. and furthermore are impermanent, it lacks constancy and is changing moment to moment. So know that all conditioned dharmas including Karma to be constantly fluctuating and empty of inherent existence, and are dependently co-arisen.
Diamond Sutra says:
Like a meteor, like darkness, as a flickering lamp, An illusion, like hoar-frost or a bubble, Like clouds, a flash of lightning, or a dream: So is all conditioned existence to be seen.Karma is action as part of dependent co-arising (it has to do with volitional activities in part of the 12 interdependent links or the 5 skhandas), it lacks inherent existence and permanency, therefore it is empty in the ultimate sense. But it is not to be misunderstood as the nihilistic idea of non-existence.
It is empty of inherent existence.
Nagarjuna vs Opponent:
[33] Opponent: The Bhagavat, the Teacher, has spoken of karma's duration, of karma's nature, and of karma's result, and also of the personal karma of living beings and of the non-destruction of karma.
[34] Reply: Karma is said to lack own-being. [Karma] that is not born is not destroyed. From that again I-making is born. But the belief that creates it is due to discrimination.
[35] If karma had own-being the body created by it would be permanent. So karma would not result in suffering and would therefore be substantial.
[36] Karma is not born from conditions and by no means from non-conditions, for karma-formations are like an illusion, a city of gandharvas, and a mirage.
[37] Karma has klesas as its cause. [Being] klesas, the karma-formations are of impassioned nature (klesatmaka). A body has karma as its cause. So [all] three are empty of own-being.
[38] Without karma, no agent. Without these two, no result. Without these, no enjoyer. Therefore things are void.
[39] When — because the truth is seen — one correctly understands that karma is empty, karma does not arise. When [karma] is no more, what arises from karma arises no more.
-----
It is important not to misunderstand [36] as saying that karma is not of dependent co-arising, but it just means that is no real 'existence' of 'karma' which is created out of various conditions.