Fix errors from assuming all room_participant rows had a non-null participant_index

Rows representing pending participants have a null participant_index.

Co-authored-by: Conrad <conrad@zed.dev>

crates/collab/src/db/tables/room_participant.rs

@@ -18,7 +18,7 @@ pub struct Model {
     pub calling_user_id: UserId,
     pub calling_connection_id: i32,
     pub calling_connection_server_id: Option<ServerId>,
-    pub participant_index: i32,
+    pub participant_index: Option<i32>,
 }
 
 impl Model {