fix don't try to cleanup dir before it exists

Author: Tristan Charbonneau

nginx/servers_config.sls

@@ -165,11 +165,13 @@ nginx_server_available_dir:
 {%   do valid_sites.append(server) %}
 {% endfor %}
 
-{% for filename in salt['file.readdir'](nginx.lookup.server_enabled) %}
-{%   if filename not in valid_sites %}
+{% if salt['file.directory_exists'](nginx.lookup.server_enabled) %}
+{%   for filename in salt['file.readdir'](nginx.lookup.server_enabled) %}
+{%     if filename not in valid_sites %}
 
 {{ nginx.lookup.server_enabled ~ '/' ~ filename }}:
   file.absent
 
-{%   endif %}
-{% endfor %}
+{%     endif %}
+{%   endfor %}
+{% endif %}