roles/nginx: Check for variable existence before testing

Some vhosts might not have WordPress so we need to be better about
testing for this before checking its value.

roles/nginx/tasks/vhosts.yml

@@ -22,13 +22,13 @@
 
 - name: Install WordPress
   git: repo=https://github.com/WordPress/WordPress.git dest={{ nginx_root_prefix }}/{{ item.domain_name }}/wordpress version={{ item.wordpress_version }} depth=1 force=yes
-  when: item.has_wordpress == True
+  when: item.has_wordpress is defined and item.has_wordpress == True
   with_items: "{{ nginx_vhosts }}"
   tags: wordpress
 
 - name: Fix WordPress directory permissions
   file: path={{ nginx_root_prefix }}/{{ item.domain_name }} state=directory owner=nginx group=nginx recurse=yes
-  when: item.has_wordpress == True
+  when: item.has_wordpress is defined and item.has_wordpress == True
   with_items: "{{ nginx_vhosts }}"
   tags: wordpress