/publicRooms causes internal server error if the server parameter matches the homeserver domain #6325

curl --request POST --header "Content-Type: application/json"  --data '{"limit":20,"include_all_networks":true}' "https://matrix.domain.tld/_matrix/client/r0/publicRooms?server=something.domain.tld&access_token=<le_token>

2019-11-04 19:52:26,074 - synapse.http.server - 109 - ERROR - POST-705775- Failed handle request via 'PublicRoomListRestServlet': <XForwardedForRequest at 0x80e5ef048 method='POST' uri='/_matrix/client/r0/publicRooms?server=domain.tld&access_token=<redacted>' clientproto='HTTP/1.1' site=8008>
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/synapse/http/server.py", line 77, in wrapped_request_handler
    await h(self, request)
  File "/usr/local/lib/python3.6/site-packages/synapse/http/server.py", line 326, in _async_render
    callback_return = await callback_return
  File "/usr/local/lib/python3.6/site-packages/synapse/rest/client/v1/room.py", line 417, in on_POST
    third_party_instance_id=third_party_instance_id,
  File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/usr/local/lib/python3.6/site-packages/synapse/handlers/room_list.py", line 391, in get_remote_public_room_list
    third_party_instance_id=third_party_instance_id,
  File "/usr/local/lib/python3.6/site-packages/synapse/handlers/room_list.py", line 441, in _get_remote_list_cached
    third_party_instance_id=third_party_instance_id,
  File "/usr/local/lib/python3.6/site-packages/synapse/util/caches/response_cache.py", line 151, in wrap
    result = self.set(key, d)
  File "/usr/local/lib/python3.6/site-packages/synapse/util/caches/response_cache.py", line 95, in set
    result = ObservableDeferred(deferred, consumeErrors=True)
  File "/usr/local/lib/python3.6/site-packages/synapse/util/async_helpers.py", line 87, in __init__
    deferred.addCallbacks(callback, errback)
AttributeError: 'NoneType' object has no attribute 'addCallbacks'