Don't require optional invite_room_state field on fed v2 invite (#14083)

Author: anoadragon453

changelog.d/14083.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where Synapse would error on the optional 'invite_room_state' field not being provided to [`PUT /_matrix/federation/v2/invite/{roomId}/{eventId}`](https://spec.matrix.org/v1.4/server-server-api/#put_matrixfederationv2inviteroomideventid).

synapse/federation/transport/server/federation.py

@@ -489,7 +489,7 @@ async def on_PUT(
 
         room_version = content["room_version"]
         event = content["event"]
-        invite_room_state = content["invite_room_state"]
+        invite_room_state = content.get("invite_room_state", [])
 
         # Synapse expects invite_room_state to be in unsigned, as it is in v1
         # API