Fix hiding devices names over federation (#10015)

And don't include blank opentracing stuff in device list updates.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

changelog.d/10015.bugfix

@@ -0,0 +1 @@
+Prevent device names from appearing in device list updates when `allow_device_name_lookup_over_federation` is `false`.

synapse/storage/databases/main/devices.py

@@ -539,9 +539,11 @@ class DeviceWorkerStore(RoomMemberWorkerStore, EndToEndKeyWorkerStore):
                     "device_id": device_id,
                     "prev_id": [prev_id] if prev_id else [],
                     "stream_id": stream_id,
-                    "org.matrix.opentracing_context": opentracing_context,
                 }
 
+                if opentracing_context != "{}":
+                    result["org.matrix.opentracing_context"] = opentracing_context
+
                 prev_id = stream_id
 
                 if device is not None:
@@ -549,7 +551,11 @@ class DeviceWorkerStore(RoomMemberWorkerStore, EndToEndKeyWorkerStore):
                     if keys:
                         result["keys"] = keys
 
-                    device_display_name = device.display_name
+                    device_display_name = None
+                    if (
+                        self.hs.config.federation.allow_device_name_lookup_over_federation
+                    ):
+                        device_display_name = device.display_name
                     if device_display_name:
                         result["device_display_name"] = device_display_name
                 else: