Allow null values for labels and ports, convert to empty lists

templates/docker-compose.yml.j2

@@ -72,12 +72,19 @@ services:
 {% endfor %}
 {% endif %}
 {% if container.labels is defined %}
+{% if container.labels is none %}
+    labels: []
+{% else %}
     labels:
 {% for label in container.labels %}
       - {{ label }}
 {% endfor %}
 {% endif %}
+{% endif %}
 {% if container.ports is defined %}
+{% if container.ports is none %}
+    ports: []
+{% else %}
     ports:
 {% for port in container.ports %}
       - {{ port }}