gvle: enables to set empty experimental condition port

[Chabrier]

gvle enables to save a vpz with empty parameters. But the vle simulator , according to the dtd, does not allow this syntax.

The solution is either to provide defaut value or the null one

[rtrepos]

Hi, I don't see any error from the parsing of this vpz. I have apprently a simulation error if I try to acces to the value, something like :

throws error in constructor: `Map: the key 'tt' have empty (null) value'

Still, the question remains : should we ban the empty port value ( <ports></ports>) ? according the dtd ? Let me know if you see an obvious choice to make.