Form's data cannot be submitted to external system if the first form element is not mapped to any remote field

Description

Steps to reproduce:

1. Create a form and map it with a remote datasourse:

2. Add some fields to the form and map them to corresponding fields in the remote datasource, EXCEPT the first field:

3. Add the form to a page, then open the page and submit the form.

Actual result: Form's data is not submitted to the external datasource
Expected result: Form's data is submitted to the external datasource