fix: skip multus when not defined (#10934)

fix task failure:
TASK [kubernetes-apps/network_plugin/multus : Multus | Start resources] ************************************************
fatal: [hfal12k8n1 -> {{ groups['kube_control_plane'][0] }}]: FAILED! => {"msg": "Error in jmespath.search in json_query filter plugin:\n'ansible.vars.hostvars.HostVarsVars object' has no attribute 'multus_manifest_2'"}