Make 'event_id' a required parameter in federated state requests (#4741)

* make 'event_id' a required parameter in federated state requests

As per the spec: https://matrix.org/docs/spec/server_server/r0.1.1.html#id40

Signed-off-by: Joseph Weston <joseph@weston.cloud>

* add changelog entry for bugfix

Signed-off-by: Joseph Weston <joseph@weston.cloud>

* Update server.py

changelog.d/4740.bugfix

@@ -0,0 +1 @@
+'event_id' is now a required parameter in federated state requests, as per the matrix spec.

synapse/federation/transport/server.py

@@ -393,7 +393,7 @@ class FederationStateServlet(BaseFederationServlet):
         return self.handler.on_context_state_request(
             origin,
             context,
-            parse_string_from_args(query, "event_id", None),
+            parse_string_from_args(query, "event_id", None, required=True),
         )
 
 
@@ -404,7 +404,7 @@ class FederationStateIdsServlet(BaseFederationServlet):
         return self.handler.on_state_ids_request(
             origin,
             room_id,
-            parse_string_from_args(query, "event_id", None),
+            parse_string_from_args(query, "event_id", None, required=True),
         )