keys/query: Omit device displayname if null

... which makes it more consistent with user displaynames.

synapse/handlers/e2e_keys.py

@@ -122,9 +122,10 @@ class E2eKeysHandler(object):
         for user_id, device_keys in results.items():
             for device_id, device_info in device_keys.items():
                 r = json.loads(device_info["key_json"])
-                r["unsigned"] = {
-                    "device_display_name": device_info["device_display_name"],
-                }
+                r["unsigned"] = {}
+                display_name = device_info["device_display_name"]
+                if display_name is not None:
+                    r["unsigned"]["device_display_name"] = display_name
                 result_dict[user_id][device_id] = r
 
         defer.returnValue(result_dict)