I think there is a mistake in the phrasing of the question. Since x is not restricted to be nonnegative the basis of the nullspace of A is not sign-restricted.
So what you have is a convex polyhedral cone. For those you need of course the extreme rays - these are the ones you are looking for. Here is a classic paper on the subject:
http://www.unc.edu/~erdavid/articles/56.pdf
But I should think that you find descriptions on this topic in any linear programming book.
Yes, those constraints define a polyhedral cone, which has only one extreme point, namely the origin. To enumerate the extreme rays you could use a software package like PORTA or cdd.