如何删除Symfony表单集合的空日期时间条目？ （delete_emply不工作）

I am using a Symfony form with a collection of Datetime entries. How can I remove the empty datetime entries from the collection when the form is submitted?

I have set delete_empty to true but it is not working. I am trying hard to follow the doc instructions (https://symfony.com/doc/current/reference/forms/types/collection.html#delete-empty) but I must be missing something somewhere. I have also tried the required option to false and empty_data to null. Should I write a callable that check if datetime is empty? If so how to check if a datetime is empty?

Here is my form builder code:

$form = $this->createFormBuilder($event)
            ->add('datetimes', CollectionType::class, [
                'entry_type' => DateTimeType::class,
                'entry_options' => [
                    'date_widget' => 'single_text',
                    'input' => 'string',
                    'time_widget' => 'single_text',
                    'html5' => false,
                    'required' => false,
                    'empty_data' => null,
                ],
                'label' => false,
                'allow_add' => true,
                'allow_delete' => true,
                'prototype' => false,
                'delete_empty' => true
            ])
            ->add('submit', SubmitType::class)
            ->getForm();

Can someone tell me what I am missing or doing wrong here?