Webhook unstable in form with date time range

Description

Preconditions

• Install a sample form.
• A Webhook URL is created.

Steps to reproduce

1. Create a form with these elements: Text, Date time range, Submit.
2. Add the webhook URL under 'trigger webhook after form submission 'section (uncheck "POST data in JSON format" option).
3. Add the form to a page then publish all.
4. Enter data, leave Datetime Range blank, then submit form

Expected: Webhook is triggered properly.
Actual: Webhook is not working.