Make sure prev_ids defaults to empty list

While this change seems a bit strange, I was seeing TypeErrors on my
homeserver because someone is sending device lists where they explicitly
set `"prev_id": null` which causes the next list comprehension to fail.

Signed-off-by: Carl Bordum Hansen <carl@bordum.dk>

synapse/handlers/device.py

@@ -762,7 +762,7 @@ async def incoming_device_list_update(
         user_id = edu_content.pop("user_id")
         device_id = edu_content.pop("device_id")
         stream_id = str(edu_content.pop("stream_id"))  # They may come as ints
-        prev_ids = edu_content.pop("prev_id", [])
+        prev_ids = edu_content.pop("prev_id") or []
         prev_ids = [str(p) for p in prev_ids]  # They may come as ints
 
         if get_domain_from_id(user_id) != origin: