Add empty list as default for adoption scenarios without networker nodes

Networker nodes are not required in all adoption topologies.
We need to provide default filter to be able to generate adoption_vars
template if osp-networkers are not defined in _vm_groups.

Fixes: #2548

roles/adoption_osp_deploy/templates/adoption_vars.yaml.j2

@@ -9,7 +9,7 @@ edpm_computes: |
   ["{{ compute }}.{{ cifmw_adoption_osp_deploy_scenario.cloud_domain }}"]="{{ node_nets.networks.ctlplane.ip_v4 }}"
   {% endfor %}
 edpm_networkers: |
-  {% for networker in _vm_groups['osp-networkers'] %}
+  {% for networker in _vm_groups['osp-networkers'] | default([]) %}
   {% set node_nets = cifmw_networking_env_definition.instances[networker] %}
   ["{{ networker }}.{{ cifmw_adoption_osp_deploy_scenario.cloud_domain }}"]="{{ node_nets.networks.ctlplane.ip_v4 }}"
   {% endfor %}