[networking_mapper] Make null templates behave like an empty one

Now, if the data inside a template network is null the mapper will raise
an unhandled exception, that may be fine, but for this particular case
making null and empty behave the same way will allow the mapper to raise
a meaningful error letting him know the field cannot be empty as some
child elements are mandatory.

plugins/module_utils/net_map/networking_definition.py

@@ -2056,7 +2056,7 @@ def __parse_raw(
         for network_name, network_data in child_nets.items():
             self.__parse_raw_net(
                 network_name,
-                network_data,
+                network_data or {},
                 network_definitions,
                 trunk_parents=set(trunk_parents.keys()),
             )