Volume backup response schemas are incorrect

Change I2ab6ecfeb0c48c7ef4a0fc94e670d43dbc3f6650 added response
schemas for volume backup responses. The schema specifies the 'name'
as type string in the response for create-backup and update-backup,
but the name is not required in the create request and the cinder
backup service does not supply one. This element should be string
or null.

Change-Id: I032e54a2065859d4ebc0969faccf68494fe745bc
Closes-bug: #1894844
diff --git a/tempest/lib/api_schema/response/volume/backups.py b/tempest/lib/api_schema/response/volume/backups.py
index 9e85f5f..cba7981 100644
--- a/tempest/lib/api_schema/response/volume/backups.py
+++ b/tempest/lib/api_schema/response/volume/backups.py
@@ -66,7 +66,7 @@
                 'properties': {
                     'id': {'type': 'string', 'format': 'uuid'},
                     'links': parameter_types.links,
-                    'name': {'type': 'string'},
+                    'name': {'type': ['string', 'null']},
                     # TODO(zhufl): metadata is added in 3.43, we should move it
                     # to the 3.43 schema file when microversion is supported
                     # in volume interfaces.
@@ -91,7 +91,7 @@
                 'properties': {
                     'id': {'type': 'string', 'format': 'uuid'},
                     'links': parameter_types.links,
-                    'name': {'type': 'string'},
+                    'name': {'type': ['string', 'null']},
                     'metadata': {'^.+$': {'type': 'string'}}
                 },
                 'additionalProperties': False,