!fixup recency timestamp

crates/matrix-sdk-base/src/rooms/normal.rs

@@ -962,6 +962,7 @@ pub struct RoomInfo {
     /// timestamp of the room. Thus, using this `recency_timestamp` value is
     /// more accurate than relying on the latest event.
     #[cfg(feature = "experimental-sliding-sync")]
+    #[serde(default)]
     pub(crate) recency_timestamp: Option<MilliSecondsSinceUnixEpoch>,
 }