style: Make "empty timeline" warning only warn in debug mode

src/api/client/sync/v3/joined.rs

@@ -272,7 +272,7 @@ async fn build_state_and_timeline(
 	// joined since the last sync, that being the syncing user's join event. if
 	// it's empty something is wrong.
 	if joined_since_last_sync && timeline.pdus.is_empty() {
-		warn!("timeline for newly joined room is empty");
+		debug_warn!("timeline for newly joined room is empty");
 	}
 
 	let (summary, device_list_updates) = try_join(