Ensure the field name is properly added to the error message when the email address is blank.

[iamdharmesh]

Description of the Change

As reported in #73, when attempting to submit the subscribe form without entering an email address, the error message did not include the field name, causing confusion about which field is incorrect and what needs to be fixed to submit the form successfully.

This PR introduces a minor fix to properly identify the field with the error, making the error message clear and specifically linked to the relevant form field.

Closes #73

How to test the Change

1. Install and set up the plugin.
2. Add the subscribe form using a block, widget, or shortcode.
3. Visit the subscription form page on the front end, and click "Subscribe" without entering an email address.
4. Ensure the error message is clear, identifies the field with the issue, and describes the related error.

Changelog Entry

Fixed - Ensure the field name is properly added to the error message when the email address is blank.

Credits

Props @jerclarke, @iamdharmesh

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.