cli: Generate empty list for empty option

Currently when a command receives an option that represents a list of
values but without any value the CLI automatically adds the string
"None" as the only value. This can cause issues. What the CLI should
generate in this cases is an empty list.

Change-Id: Ic95dadcbfd7f77b49524df0ffd05dcdcf7af8f1e
Bug-Url: https://bugzilla.redhat.com/1085463
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>