client-api: Account for knocked rooms in Rooms::is_empty

The `rooms` field is skipped at serialization when `Rooms::is_empty`
returns true. The method omitted `knock`, so a sync whose only updates
are knocked rooms dropped the entire `rooms` object, leaving the
knocker's own knock absent from /sync.

Signed-off-by: Jason Volk <jason@zemos.net>

Author: jevolk

crates/ruma-client-api/src/sync/sync_events/v3.rs

@@ -213,7 +213,10 @@ impl Rooms {
 
     /// Returns true if there is no update in any room.
     pub fn is_empty(&self) -> bool {
-        self.leave.is_empty() && self.join.is_empty() && self.invite.is_empty()
+        self.leave.is_empty()
+            && self.join.is_empty()
+            && self.invite.is_empty()
+            && self.knock.is_empty()
     }
 }