Prevent `null` Sass variables from creating empty CSS variables

Created by: Kiriakk

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

I'm trying to compile bootstrap's scss without any changes. It compiles fine, but produces broken css with 15 absent values. For example this: --bs-heading-color: ; at line 71

Compiling with Dart-Sass 1.52.3

Actually, already compiled css, which ships inside bootstrap-5.2.0-beta1.zip and latest bootstrap-main.zip files contain that broken css as well.

Scss from 5.1.3 compiles proper css on the same setup.

Reduced test cases

Try to compile with minimal custom.scss file: @import "../node_modules/bootstrap/scss/bootstrap";

What operating system(s) are you seeing the problem on?

Windows

What browser(s) are you seeing the problem on?

Chrome, Opera

What version of Bootstrap are you using?

5.2.0 beta 1, latest snapshot.