Opt-in form does not work

[kaittodesk]

Bug description
Module's opt-in form does not submit data to Smaily. It does make an AJAX request, but subscriber is not added to Smaily.

Steps for reproducing
Steps to reproduce the behavior:

1. Transplant "Smaily for PrestaShop" to "displayFooterBefore" position.
2. Go to the public page.
3. Scroll down to the bottom of the page.
4. Fill out the Smaily opt-in form.
5. Submit the form.

Expected behavior
CAPTCHA confirmation page is shown.

Actual behavior
An AJAX request is made and empty success message is shown.

Screenshots

Versions
PrestaShop: 1.7.6.8
Smaily for PrestaShop: 1.5.0

Additional context
The issue is caused by the new version of PrestaShop's ps_emailsubscription module (version 2.6.0) - PrestaShop/ps_emailsubscription#48