Allow image name to be empty for simple list test

An empty name is valid for an image, but the list_images scheme
is missing the null option, so test_list_images test fails.

This commit brings the list_images scheme in line with the
list_images_detailed schema and allows the name field to be empty,
and both test_list_images and test_list_images_with_detail tests
pass.

Change-Id: I40f05fc02c199747065830f7a7719680bbaa8d71
diff --git a/tempest/lib/api_schema/response/compute/v2_1/images.py b/tempest/lib/api_schema/response/compute/v2_1/images.py
index daab898..b0f1934 100644
--- a/tempest/lib/api_schema/response/compute/v2_1/images.py
+++ b/tempest/lib/api_schema/response/compute/v2_1/images.py
@@ -77,7 +77,7 @@
                     'properties': {
                         'id': {'type': 'string'},
                         'links': image_links,
-                        'name': {'type': 'string'}
+                        'name': {'type': ['string', 'null']}
                     },
                     'additionalProperties': False,
                     'required': ['id', 'links', 'name']