Fix bug with null values in configuration (#1305)

Fixes #1281.

Author: jathak

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 1.32.13
 
+* **Potentially breaking bug fix:** Null values in `@use` and `@forward`
+  configurations no longer override the `!default` variable, matching the
+  behavior of the equivalent code using `@import`.
+
 * Use the proper parameter names in error messages about `string.slice`
 
 ## 1.32.12

lib/src/visitor/async_evaluate.dart

@@ -1918,7 +1918,7 @@ class _EvaluateVisitor
     if (node.isGuarded) {
       if (node.namespace == null && _environment.atRoot) {
         var override = _configuration.remove(node.name);
-        if (override != null) {
+        if (override != null && override.value != sassNull) {
           _addExceptionSpan(node, () {
             _environment.setVariable(
                 node.name, override.value, override.assignmentNode,

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: acaafcfe17e8cb582fb01ea8b95afaf871af4eed
+// Checksum: 648f9a2041613a8f11c46986b4b4d4ccbaa0c489
 //
 // ignore_for_file: unused_import
 
@@ -1910,7 +1910,7 @@ class _EvaluateVisitor
     if (node.isGuarded) {
       if (node.namespace == null && _environment.atRoot) {
         var override = _configuration.remove(node.name);
-        if (override != null) {
+        if (override != null && override.value != sassNull) {
           _addExceptionSpan(node, () {
             _environment.setVariable(
                 node.name, override.value, override.assignmentNode,

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.32.13-dev
+version: 1.32.13
 description: A Sass implementation in Dart.
 author: Sass Team
 homepage: https://github.com/sass/dart-sass