Call to void native method returns null instead of void

[javierfernandes]

Related to #845

This is for this particular case in the REPL

Wollok interactive console (type "quit" to quit):
>>> const list = [1,2,3]
[1, 2, 3]
>>> list.remove(1)
null
>>> 

In this example Collection.remove() is a void method.

The output of the program in the repl is

Wollok interactive console (type "quit" to quit):
>>> const list = [1,2,3]
[1, 2, 3]
>>> list.remove(1)
null
>>> 

[javierfernandes]

Fixed !

The output now is

Wollok interactive console (type "quit" to quit):
>>> const list = [1,2,3]
[1, 2, 3]
>>> list.remove(1)
>>> 

So.. nothing gets printed