Fix conditional generating new certificate

Author: enricostano

tasks/certificate.yml

@@ -5,6 +5,5 @@
   register: letsencrypt_cert
 
 - name: Generate new certificate if one doesn't exist
-  shell: "certbot certonly --nginx --email '{{ letsencrypt_email }}' --agree-tos -d '{{ domain_name }}' {% if certbot_nginx_cert_name %} --cert-name '{{ certbot_nginx_cert_name }}' {% endif %} {% if letsencrypt_staging %} --staging {% endif %}"
+  shell: "certbot certonly --nginx --email '{{ letsencrypt_email }}' --agree-tos -d '{{ domain_name }}' {% if certbot_nginx_cert_name is defined %} --cert-name '{{ certbot_nginx_cert_name }}' {% endif %} {% if letsencrypt_staging %} --staging {% endif %}"
   when: not letsencrypt_cert.stat.exists
-