fix: last_activity_at is nil when conv is created

The payload does not include last_activity_at when the conversation is created.
Becuase of this the frontend is not able to sort the conversations when appending this.
Another problem is that the last_activity_at is not always present, it is added only when a message is created, and it updates it. So this can be nil when the conversation is created, so we fallback to created_at only at the presentation layer

app/presenters/conversations/event_data_presenter.rb

@@ -40,6 +40,7 @@ class Conversations::EventDataPresenter < SimpleDelegator
     {
       agent_last_seen_at: agent_last_seen_at.to_i,
       contact_last_seen_at: contact_last_seen_at.to_i,
+      last_activity_at: last_activity_at ? last_activity_at.to_i : created_at.to_i,
       timestamp: last_activity_at.to_i,
       created_at: created_at.to_i
     }