sliding sync: fix unsetting the avatar in a sliding sync room

The meaning of "null" and "undefined" were conflated by Ruma, previously. This updates to the latest Ruma, which changed the `avatar` type from
`Option` to `JsOption` and thus allowed us to distinguish both cases: `null` means the avatar has been unset, `undefined` means it's not changed
since the previous request.

Cargo.lock

@@ -4676,9 +4676,9 @@ dependencies = [
 
 [[package]]
 name = "ruma-client-api"
-version = "0.17.3"
+version = "0.17.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2428ee1488551cbc2bc4ef936c9452ac35ccd5c7e4e4df12c54b67afa6b262fb"
+checksum = "641837258fa214a70823477514954ef0f5d3bc6ae8e1d5d85081856a33103386"
 dependencies = [
  "assign",
  "bytes",

crates/matrix-sdk/src/sliding_sync/room.rs

@@ -78,7 +78,7 @@ impl SlidingSyncRoom {
     pub fn avatar_url(&self) -> Option<OwnedMxcUri> {
         let inner = self.inner.inner.read().unwrap();
 
-        inner.avatar.clone()
+        inner.avatar.clone().into_option()
     }
 
     /// Is this a direct message?
@@ -167,7 +167,10 @@ impl SlidingSyncRoom {
                 inner.name = name;
             }
 
-            if avatar.is_some() {
+            // Note: in the server specification, the avatar is undefined when it hasn't
+            // changed, and it's set to null when it's been unset, so we
+            // distinguish the two here.
+            if !avatar.is_undefined() {
                 inner.avatar = avatar;
             }
 
@@ -321,7 +324,7 @@ mod tests {
     use matrix_sdk_test::async_test;
     use ruma::{
         api::client::sync::sync_events::v4, assign, events::room::message::RoomMessageEventContent,
-        mxc_uri, room_id, serde::Raw, uint, RoomId,
+        mxc_uri, room_id, serde::Raw, uint, JsOption, RoomId,
     };
     use serde_json::json;
     use wiremock::MockServer;
@@ -906,7 +909,7 @@ mod tests {
                 v4::SlidingSyncRoom::default(),
                 {
                     name: Some("foobar".to_owned()),
-                    avatar: Some(mxc_uri!("mxc://homeserver/media").to_owned()),
+                    avatar: JsOption::Some(mxc_uri!("mxc://homeserver/media").to_owned()),
                 }
             ),
             timeline_queue: vector![TimelineEvent::new(