fix: Skip contact_last_seen_at in empty conversation payload (#4387)

* Fix undefined `last_seen_at` empty conversation

* chore: remove the comment

Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/views/api/v1/widget/messages/index.json.jbuilder

@@ -12,5 +12,5 @@ json.payload do
   end
 end
 json.meta do
-  json.contact_last_seen_at @conversation.contact_last_seen_at.to_i
+  json.contact_last_seen_at @conversation.contact_last_seen_at.to_i if @conversation.present?
 end