Check if openstack application credentials are empty since they always exists (#8021)

roles/kubernetes-apps/external_cloud_controller/openstack/templates/external-openstack-cloud-config.j2

 [Global]
 auth-url="{{ external_openstack_auth_url }}"
-{% if external_openstack_application_credential_id is not defined and external_openstack_application_credential_name is not defined %}
+{% if external_openstack_application_credential_id == "" and external_openstack_application_credential_name == "" %}
 username="{{ external_openstack_username }}"
 password="{{ external_openstack_password }}"
 {% endif %}