fix(css): crash when CSS @import is ignored with exportType: "style" #22842

Problem

When using native CSS (experiments.css: true) with parser.exportType: "style", @import rules that point to an ignored module (e.g., via IgnorePlugin or resolve.alias: { "...": false }) cause webpack to throw TypeError: Cannot read properties of null (reading 'userRequest') during code generation.

Current Behavior

Building a project that contains a CSS file with @import "./ignored.css"; and exportType: "style", combined with new webpack.IgnorePlugin({ resourceRegExp: /ignored\.css$/ }), crashes during code generation:

TypeError: Cannot read properties of null (reading 'userRequest')
    at CssGenerator._generateImportExpressions (webpack/lib/css/CssGenerator.js:861:25)
    at generateImportCode (webpack/lib/css/CssGenerator.js:364:20)
    at CssGenerator.generate (webpack/lib/css/CssGenerator.js:558:9)

Expected Behavior

The compilation should succeed. If an @import dependency is ignored or has no resolved module (moduleGraph.getModule(dep) is null), CssGenerator._generateImportExpressions should skip it without crashing, matching the behavior in CssGenerator._renderMergedCss.

Reproduction

1. Enable experiments.css: true.
2. Configure a CSS rule with exportType: "style":

{
  test: /\.css$/,
  type: "css/auto",
  parser: { exportType: "style" }
}

3. Add new webpack.IgnorePlugin({ resourceRegExp: /ignored\.css$/ }).
4. Import a CSS file containing @import "./ignored.css";.
5. Run webpack build -> crashes with TypeError: Cannot read properties of null (reading 'userRequest').

Technical Context

In lib/css/CssGenerator.js, _generateImportExpressions iterates through module.dependencies for css-import dependencies:

for (const dep of module.dependencies) {
	if (!isDependencyKind(dep, "css-import")) continue;
	const depModule = /** @type {CssModule & BuiltCssModule} */ (
		moduleGraph.getModule(dep)
	);
	if (concatenationScope && concatenationScope.isModuleInScope(depModule)) {
		continue;
	}
	parts.push(
		generateContext.runtimeTemplate.moduleExports({
			module: depModule,
			chunkGraph: generateContext.chunkGraph,
			request: depModule.userRequest,
			weak: false,
			runtimeRequirements: generateContext.runtimeRequirements
		})
	);
}

When dep resolves to false (ignored), moduleGraph.getModule(dep) returns null. Accessing depModule.userRequest causes an unhandled TypeError. Adding if (!depModule) continue; resolves this issue, consistent with _renderMergedCss.