Fix a TypeError in notification_sample_base.py

In notification_sample_base, a possible error is found and fixed, as

    extra_params['return_reservation_id'] = True

is a valid code only when extra_params is not None.
However, this is not checked. So when we try to boot a server
without extra parameters, a TypeError is raised:

    TypeError: 'NoneType' object does not support item assignment

In this patchset, the possible bug is fixed.

Change-Id: I80e2286b231da39b8c1e4225145422ac48daee68

nova/tests/functional/notification_sample_tests/notification_sample_base.py

@@ -152,9 +152,8 @@ class NotificationSampleTestBase(test.TestCase,
             image_uuid='155d900f-4e14-4e4c-a73d-069cbf4541e6',
             flavor_id=flavor_id)
 
-        extra_params['return_reservation_id'] = True
-
         if extra_params:
+            extra_params['return_reservation_id'] = True
             server.update(extra_params)
 
         post = {'server': server}