Correct mistakes in the ConstructibleSet( I, J ) method, where I is an ideal and J a list of polynomials

The method used to incorrectly set the defining ideal.

The method also used to test for IsEmpty, which is automatically set by
the _ConstructibleSet function.
Correct the ConstructibleSet( X, J ) constructor, where X is an affine set and J is a list of polynomials.

The method used to use QuasiAffineSet, while ConstructibleSet is correct.