Handle zero ideal cases

[rprebet]

Handle the cases where a GB of the zero ideal is output. More precisely when

• the input polynomials array is empty or contains only zeros;
• the elimination ideal is the zero ideal.

This assumes the convention that the GB of the zero ideal is the empty set, following Cox, Little & O'Shea.

[rprebet]

The tests seems to require the GB of the zero ideal to be [0] so I adapted the code.
This cannot be done however, whenever the input ideal generators is an empty list.