ssl-keys: explicitly check for variable being defined and non-empty

new ansible is bitching about
    {% if webmaster@ubuntu-it.org %} True {% else %} False {% endif %}
being non valid or something.
Giving it a full real explicit check seems to do the trick.
Signed-off-by: Mattia Rizzolo <mapreri@ubuntu.com>

roles/ssl-keys/tasks/main.yml

@@ -9,4 +9,4 @@
   when: not letsencrypt_email
 
 - include_tasks: letsencrypt.yml
-  when: letsencrypt_email
+  when: letsencrypt_email is defined and letsencrypt_email != ""