fix(no-ununsed-class-name): fixed an error with @use at-rules (#527)

Author: marekdedic

.changeset/smart-gifts-shake.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-svelte": patch
+---
+
+fix(no-ununsed-class-name): fixed an error with @use at-rules

src/rules/no-unused-class-name.ts

@@ -116,7 +116,10 @@ function findClassesInPostCSSNode(node: AnyNode): string[] {
     )
     return classes
   }
-  if (node.type === "root" || node.type === "atrule") {
+  if (
+    (node.type === "root" || node.type === "atrule") &&
+    node.nodes !== undefined
+  ) {
     return node.nodes.flatMap(findClassesInPostCSSNode)
   }
   return []