software node: Allow node creation without properties

Software nodes are not forced to have device properties.
Adding check to property_entries_dup() to make it possible
to create software nodes that don't have any properties.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>