importutils: fix None values error

• Improves importutils when it does not discover any module inside the given package or if the package does not exists.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>