Remove Default impl for ConnectionId (#7452)

We noticed the following message in my logs when trying to debug some
lag when collaborating:

```
2024-02-06T09:42:09-08:00 [ERROR] error handling message. client_id:3, sender_id:Some(PeerId { owner_id: 327, id: 1123430 }), type:GetCompletions, error:no such connection: 0/0
```

That `0/0` looks like a bogus connection id, constructed via a derived
`Default`. We didn't ever find a code path that would *use* a default
`ConnectionId` and lead to this error, but it did seem like an
improvement to not have a `Default` for that type.

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/src/db.rs

@@ -692,7 +692,7 @@ impl ProjectCollaborator {
 pub struct LeftProject {
     pub id: ProjectId,
     pub host_user_id: UserId,
-    pub host_connection_id: ConnectionId,
+    pub host_connection_id: Option<ConnectionId>,
     pub connection_ids: Vec<ConnectionId>,
 }