"Login form username title" setting saves as "0" and causes error when override is enabled

Problem/Motivation

When enabling the "Override login form username title and description" setting in the miniorange_2fa module, and entering any custom string into the "Login form username title" field, the value is not saved correctly. It is always stored as the string "0" regardless of the input.

As a result, when viewing the login form, a fatal error occurs:
$string ("0") must be a string.

This makes the login form unusable.

Disabling the override option avoids the error, but we want to customize the form text to make it clearer for users. This is a blocking issue and needs to be fixed urgently.

Steps to reproduce

Navigate to Configuration > People > miniOrange Two Factor Settings.

Enable "Override login form username title and description".

Enter any string (e.g., Please enter your username) into "Login form username title".

Save the configuration.

Visit the login page.

Observe the error:
$string ("0") must be a string.

Proposed resolution

Ensure that the value entered into "Login form username title" is correctly stored and retrieved as a valid string. Prevent any unintended type casting or overwriting with "0".

Remaining tasks

Investigate the form submission handler and config saving logic.

Correct the data handling for this specific field.

Add validation to ensure "0" is not unintentionally used or stored.

Add tests to prevent regressions.

User interface changes

None.

API changes

None.

Data model changes

None.