Added GenericShowFields instances for NoConstructors and NoArguments

[kejace]

We needed these in order to support

derive instance genericA :: G.Generic A _

instance showGenericA :: Show A where
  show = G.genericShow

I'm not sure ["{}"] is the right thing to return here though, so I'm open to suggestions.

[paf31]

I understand why NoArguments is needed, but not NoConstructors. Also, {} is already rendered above where genericShowFields is called, so really you should only need an empty array.

[kejace]

Agreed. I changed the PR accordingly.

[paf31]

Thanks!