Is Nibbana devoid of a permanent and eternal substance? I read the following on the internet:


  According to Wikipedia by the way, Nagarjuna wrote, in the
  Mūlamadhyamakakārikā, "[A]ll experienced phenomena are empty (sunya).
  This did not mean that they are not experienced and, therefore,
  non-existent; only that they are devoid of a permanent and eternal
  substance (svabhava) because, like a dream, they are mere projections
  of human consciousness. Since these imaginary fictions are
  experienced, they are not mere names (prajnapti).


Is Nibbana devoid of a permanent and eternal substance? 

Also, is Nibbana a mere projection of human consciousness?

Also, are all things "mere names"?
From this page, according to Nagarjuna's MMK, XXIV, 18 - 19:


  
  Whatever is relativity, we proclaim that emptiness. It is dependent designation. It is also the central way.
  Nothing whatsoever is found which is not relativistically originated. Therefore, nothing whatsoever is found which is not empty.
  


A fish spends all its time in water, but may not understand what water means, unless it leaves it momentarily.

Similarly, from my understanding of Nagarjuna, if you have no samsara, then no one would understand what Nirvana means. Nirvana is identified only to contrast it with samsara. Hence, Nirvana is empty of an intrinsic essence, because its definition is relativistic i.e. relative to samsara.

Also, from this page:


  Nevertheless to assert that all things are empty of any intrinsic
  reality, for Nāgārjuna, is not to undermine the existential status of
  things as simply nothing. On the contrary, Nāgārjuna argues, to assert
  that the things are empty of any intrinsic reality is to explain the
  way things really are as causally conditioned phenomena
  (pratītyasamputpaṅhā).


This means Nagarjuna does not say that Nirvana does not exist or is nothing. Rather he says that it depends on something else. Nirvana does not substantially depend on samsara i.e. it is not conditioned or compounded by anything. However, it depends on samsara for its relativistic definition i.e. Nirvana depends on samsara semantically, rather than substantially.

Another analogy is this - if a tree falls in a forest, but there is no one there to listen to it, does it still make a sound? The sound of a tree falling is only defined relative to a listener. If there is no listener, then it does not make sense to identify sound as such. But it does not mean that it doesn't exist. Similarly, in the case of Nirvana, it is defined relative to samsara.

Nagarjuna is really discussing the mental concept of Nirvana, rather than Nirvana itself.