Fix bug 2399: vnf instantiation parameters could have no vdu params

This change fixes bug 2399 that was reported in relation a multi-site
NS deployment. However, this issue could appear in multiple
circumstances, whenever there are vnf instantiation parameters and there
are no vdu params inside.

Change-Id: Id6d3c0b88a2a8478e1908372b24e3d13cdb94dcf
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>