events: make allow field for restricted rooms optional

In the spec, it doesn't state that it is a required field, even for restricted rooms

crates/ruma-events/CHANGELOG.md

@@ -6,6 +6,7 @@ Bug fixes:
   fragment.
 - Fix serialization of `room::message::Relation` and `room::encrypted::Relation`
   which could cause duplicate `rel_type` keys. 
+- `Restricted` no longer fails to deserialize when the `allow` field is missing
 
 Improvements:
 

crates/ruma-events/src/room/join_rules.rs

@@ -168,6 +168,7 @@ impl From<JoinRule> for SpaceRoomJoinRule {
 #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
 pub struct Restricted {
     /// Allow rules which describe conditions that allow joining a room.
+    #[serde(default)]
     pub allow: Vec<AllowRule>,
 }
 
@@ -324,6 +325,16 @@ mod tests {
         assert_eq!(serde_json::to_string(&allow_rule).unwrap(), json);
     }
 
+    #[test]
+    fn restricted_room_no_allow_field() {
+        let json = r#"{"join_rule":"restricted"}"#;
+        let join_rules: RoomJoinRulesEventContent = serde_json::from_str(json).unwrap();
+        assert_matches!(
+            join_rules,
+            RoomJoinRulesEventContent { join_rule: JoinRule::Restricted(_) }
+        );
+    }
+
     #[test]
     fn join_rule_to_space_room_join_rule() {
         assert_eq!(SpaceRoomJoinRule::Invite, JoinRule::Invite.into());