Added return statement to retype_volume in v2 volumes_client

This commit adds a missing return statement to the retype_volume API
endpoint in the v2 volumes_client. Otherwise, None, instead of an
empty dict, is returned for the response body.

Closes-Bug: #1703997
Partially Implements: blueprint tempest-lib-missing-test-coverage
Change-Id: I1a308f486fd7b14a9111a3433284e3c4abf65bd3

tempest/lib/services/volume/v2/volumes_client.py

@@ -318,6 +318,7 @@ class VolumesClient(base_client.BaseClient):
         post_body = json.dumps({'os-retype': kwargs})
         resp, body = self.post('volumes/%s/action' % volume_id, post_body)
         self.expected_success(202, resp.status)
+        return rest_client.ResponseBody(resp, body)
 
     def force_detach_volume(self, volume_id, **kwargs):
         """Force detach a volume.

tempest/releasenotes/notes/add-return-value-to-retype-volume-a401aa619aaa2457.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  Add a missing return statement to the retype_volume API in the v2 volumes_client library.
+  This changes the response body from None to an empty dictionary.
+

tempest/tests/lib/services/volume/v2/test_volumes_client.py

@@ -33,6 +33,22 @@ class TestVolumesClient(base.BaseServiceTest):
                                                    'volume',
                                                    'regionOne')
 
+    def _test_retype_volume(self, bytes_body=False):
+        kwargs = {
+            "new_type": "dedup-tier-replication",
+            "migration_policy": "never"
+        }
+
+        self.check_service_client_function(
+            self.client.retype_volume,
+            'tempest.lib.common.rest_client.RestClient.post',
+            {},
+            to_utf=bytes_body,
+            status=202,
+            volume_id="a3be971b-8de5-4bdf-bdb8-3d8eb0fb69f8",
+            **kwargs
+        )
+
     def _test_force_detach_volume(self, bytes_body=False):
         kwargs = {
             'attachment_id': '6980e295-920f-412e-b189-05c50d605acd',
@@ -71,3 +87,9 @@ class TestVolumesClient(base.BaseServiceTest):
 
     def test_show_volume_metadata_item_with_bytes_body(self):
         self._test_show_volume_metadata_item(bytes_body=True)
+
+    def test_retype_volume_with_str_body(self):
+        self._test_retype_volume()
+
+    def test_retype_volume_with_bytes_body(self):
+        self._test_retype_volume(bytes_body=True)