css-tree: WalkContext.atrule can be null (#37119)

* css-tree: WalkContext.atrule can be null

* css-tree: everything except `root` can be null

Thanks @erik-kallen for the suggestion.

* css-tree: add some more type assertions to the tests

types/css-tree/css-tree-tests.ts

@@ -16,16 +16,20 @@ csstree.walk(ast, {});
 
 csstree.walk(ast, {
     enter(node, item, list) {
-        this.stylesheet; // $ExpectType StyleSheet
+        this.root; // $ExpectType CssNode
+        this.stylesheet; // $ExpectType StyleSheet | null
         node; // $ExpectType CssNode
         item; // $ExpectType ListItem<CssNode>
         list; // $ExpectType List<CssNode>
+        this.atrule; // $ExpectType Atrule | null
     },
     leave(node, item, list) {
-        this.stylesheet; // $ExpectType StyleSheet
+        this.root; // $ExpectType CssNode
+        this.stylesheet; // $ExpectType StyleSheet | null
         node; // $ExpectType CssNode
         item; // $ExpectType ListItem<CssNode>
         list; // $ExpectType List<CssNode>
+        this.atrule; // $ExpectType Atrule | null
     },
     visit: 'ClassSelector',
     reverse: false,

types/css-tree/index.d.ts

@@ -517,14 +517,14 @@ export function generate(ast: CssNode, options?: GenerateOptions): string;
 
 export interface WalkContext {
     root: CssNode;
-    stylesheet: StyleSheet;
-    atrule: Atrule;
-    atrulePrelude: AtrulePrelude;
-    rule: Rule;
-    selector: SelectorList;
-    block: Block;
-    declaration: Declaration;
-    function: FunctionNode | PseudoClassSelector | PseudoElementSelector;
+    stylesheet: StyleSheet | null;
+    atrule: Atrule | null;
+    atrulePrelude: AtrulePrelude | null;
+    rule: Rule | null;
+    selector: SelectorList | null;
+    block: Block | null;
+    declaration: Declaration | null;
+    function: FunctionNode | PseudoClassSelector | PseudoElementSelector | null;
 }
 
 export type EnterOrLeaveFn = (this: WalkContext, node: CssNode, item: ListItem<CssNode>, list: List<CssNode>) => void;