Define a sane default in case instance_name is not defined

tasks/main.yml

@@ -1,17 +1,17 @@
 ---
 - name: check if the config already exists
   ansible.builtin.stat:
-    path: "/etc/nginx/http.d/{{ instance_name }}.conf"
+    path: "/etc/nginx/http.d/{{ instance_name | default(ansible_play_name) }}.conf"
     get_mime: false
     get_checksum: false
     get_attributes: false
   when: onlyifnotexist is defined and onlyifnotexist
   register: cnf
 
-- name: configure nginx for {{ instance_name }}
+- name: configure nginx for {{ instance_name | default(ansible_play_name) }}
   template:
     src: nginx.conf.j2
-    dest: "/etc/nginx/http.d/{{ instance_name }}.conf"
+    dest: "/etc/nginx/http.d/{{ instance_name | default(ansible_play_name) }}.conf"
     mode: 0644
   when: onlyifnotexist is not defined or not onlyifnotexist or not cnf.stat.exists
   notify: