explode deprecation in PHP 8.1

Problem/Motivation

explode() PHP error message occurs in the subtheme config settings when using PHP 8.1.

Steps to reproduce

1. Launch a local site using PHP 8.1.
2. Create a subtheme using bootstrap5 config settings.
3. Install the subtheme.
4. Go to the subtheme settings.
5. The error message will appear.

Proposed resolution

Use the null coalescing operator, introduced in PHP 7, to default the value to an empty string if the value is null.