Fix _data type in notifications.

Change-Id: Idaf1d16fb8173e3841715ea64179c87ccf5efbf5
diff --git a/oslo_templates/files/queens/oslo/messaging/_notifications.conf b/oslo_templates/files/queens/oslo/messaging/_notifications.conf
index 8a86cbf..c9e130e 100644
--- a/oslo_templates/files/queens/oslo/messaging/_notifications.conf
+++ b/oslo_templates/files/queens/oslo/messaging/_notifications.conf
@@ -7,7 +7,11 @@
 # messaging, messagingv2, routing, log, test, noop (multi valued)
 # Deprecated group/name - [DEFAULT]/notification_driver
 #driver =
+{%- if _data is mapping %}
 driver = {{ _data.get('driver', 'messagingv2') }}
+{%- elif _data %}
+driver=messagingv2
+{%- endif %}
 
 # A URL representing the messaging driver to use for notifications. If
 # not set, we fall back to the same configuration used for RPC.
@@ -19,7 +23,7 @@
 # Deprecated group/name - [rpc_notifier2]/topics
 # Deprecated group/name - [DEFAULT]/notification_topics
 #topics = notifications
-{%- if _data.topics is defined %}
+{%- if _data is mapping and _data.topics is defined %}
 topics = {{ _data.topics }}
 {%- endif %}