Fix empty class list bug in node definition.

Change-Id: I666df4f45e82fbefb78228f35aa0b2e0c839b254
Related: PROD-19507
diff --git a/reclass/files/node.yml b/reclass/files/node.yml
index a916432..f10630e 100644
--- a/reclass/files/node.yml
+++ b/reclass/files/node.yml
@@ -1,5 +1,5 @@
 classes:
-{%- for class in node.classes %}
+{%- for class in node.get('classes', []) %}
 - {{ class }}
 {%- endfor %}
 parameters: