Fix forwarding for empty statement
Fix forwarding for empty statement

* in case of : if foo(): i = 2; print i
  `i` will be forwarded and the if statement will contain nothing. It won't be a valid ast.

pombredanne/pythran,pbrunet/pythran,pombredanne/pythran,pbrunet/pythran,hainm/pythran,hainm/pythran,artas360/pythran,hainm/pythran,artas360/pythran,serge-sans-paille/pythran,pbrunet/pythran,serge-sans-paille/pythran,artas360/pythran,pombredanne/pythran