Classes settings page crashes if classes.region or classes.field config is missing

Problem/Motivation

If the configuration keys classes.region or classes.field do not exist, the Classes settings page becomes unusable and triggers a fatal error.

The following errors are thrown:

TypeError: implode(): If argument #1 ($separator) is of type string, argument #2 ($array) must be of type array, null given in implode() (line 29 of modules/contrib/ds/src/Form/ClassesForm.php).

TypeError: implode(): If argument #1 ($separator) is of type string, argument #2 ($array) must be of type array, null given in implode() (line 36 of modules/contrib/ds/src/Form/ClassesForm.php).

It is unclear whether this situation (missing configuration for these keys) is expected or how to reliably reproduce it. The issue was observed on an existing project that had been taken over, where the configuration appeared to be incomplete or missing.

Steps to reproduce

1. Have a project where the configuration keys classes.region and/or classes.field are missing or set to null.
2. Go to the Classes settings page.
3. Observe the fatal TypeError caused by implode() receiving null instead of an array.

Proposed resolution

Add a safeguard in ClassesForm.php to ensure that classes.region and classes.field default to an empty array when the configuration is missing or null.

For example, ensure that the value passed to implode() is always an array by providing an empty array as a fallback. This would prevent the fatal error and make the form more robust in case of incomplete or corrupted configuration.