fix: since we don't support optionals return empty array (#519)

This fixes and issue with the online_boutique sample app crashing on a
`null` array

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

examples/online-boutique/services/cart/store.go

@@ -49,7 +49,7 @@ func (s *Store) Get(userID string) []Item {
 	defer s.lock.Unlock()
 	items, ok := s.carts.Get(userID)
 	if !ok {
-		return nil
+		return []Item{}
 	}
 	out := make([]Item, len(items))
 	copy(out, items)