Format datetime in resource or collection with:
// Error with null
'published_at' => $this->published_at->format('Y-m-d H:i:s'),
// No errors
'published_at' => $this->published_at != null ? $this->published_at->format('Y-m-d H:i:s') : null,
// Custom date if null
'published_at' => ($this->published_at ?? now()->addDays(10))->format('Y-m-d H:i:s'),
Regards