Don't show user-data when its not sent.

Don't show user-data in the instance metadata service
if the instance was not started with any user-data.

LP: #1035317

Change-Id: Idf9b263a39f3b8d75aaf3e6d1debbe283cf9ff64
Signed-off-by: Chuck Short <chuck.short@canonical.com>

nova/compute/api.py

@@ -462,7 +462,7 @@ def _create_instance(self, context, instance_type,
             'ephemeral_gb': instance_type['ephemeral_gb'],
             'display_name': display_name,
             'display_description': display_description or '',
-            'user_data': user_data or '',
+            'user_data': user_data,
             'key_name': key_name,
             'key_data': key_data,
             'locked': False,