try another way to check if docker_mirror is not empty

ansible/roles/pre-configure/tasks/main.yml

@@ -104,6 +104,10 @@
 - name: Configure firewall
   import_tasks: firewall.yml
 
+- name: debug docker_mirror data
+  debug:
+    msg: Docker mirror is "{{ docker_mirror }}"
+
 - name: Write docker registries.yaml
   tags:
     - k3s
@@ -112,4 +116,4 @@
     src: registries.yaml
     dest: /etc/rancher/k3s/registries.yaml
     mode: '0740'
-  when: docker_mirror != "{}"
+  when: docker_mirror is not empty