/createRoom sends join_rules event without join_rule

[timokoesters]

I see this request in the network debugger when creating a private room:
{"name":"asdf","preset":"private_chat","visibility":"private","initial_state":[{"type":"m.room.guest_access","state_key":"","content":{"guest_access":"can_join"}},{"type":"m.room.encryption","state_key":"","content":{"algorithm":"m.megolm.v1.aes-sha2"}},{"type":"m.room.join_rules","content":{}}]}

As you can see, the join_rules event has an empty content map, but in https://spec.matrix.org/unstable/client-server-api/#mroomjoin_rules the join_rule field is required.

Version information

Version: Current staging.element.io