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.
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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.