Fix sonic-mgmt testbed-cli interface_to_vms undefined failure. (#12890)

What is the motivation for this PR?
This change is to fix the error above, when VM list is empty.

How did you do it?
The fix is straightforward - to give it a default value that is an empty list when enumerating.

How did you verify/test it?
The change is run locally with topology with empty VMs, and it works.

ansible/config_sonic_basedon_testbed.yml

@@ -262,7 +262,7 @@
     set_fact:
       intf_names: "{{ intf_names | default({}) | combine({item.0.name:  intf_names[item.0.name]|default([]) + [ port_alias[item.1]] }) }}"
     with_subelements:
-      - "{{ interface_to_vms }}"
+      - "{{ interface_to_vms | default([]) }}"
       - "ports"
     when: "'cable' not in topo"
 
@@ -272,7 +272,7 @@
       vm_asic_ifnames: "{{ vm_asic_ifnames | default({}) | combine({item.0.name: vm_asic_ifnames[item.0.name]|default([]) + [ front_panel_asic_ifnames[item.1]] }) }}"
       vm_asic_ids: "{{ vm_asic_ids | default({}) | combine({item.0.name: vm_asic_ids[item.0.name]|default([]) + [ front_panel_asic_ifs_asic_id[item.1]] }) }}"
     with_subelements:
-      - "{{ interface_to_vms }}"
+      - "{{ interface_to_vms | default([]) }}"
       - "ports"
     when: front_panel_asic_ifnames != []