refactor(@angular-devkit/build-webpack): adjust callback type to support webpack 5

Author: clydin

packages/angular_devkit/build_webpack/src/webpack/index.ts

@@ -52,11 +52,15 @@ export function runWebpack(
 
   return createWebpack(config).pipe(
     switchMap(webpackCompiler => new Observable<BuildResult>(obs => {
-      const callback = (err: Error | undefined, stats: webpack.Stats) => {
+      const callback = (err?: Error, stats?: webpack.Stats) => {
         if (err) {
           return obs.error(err);
         }
 
+        if (!stats) {
+          return;
+        }
+
         // Log stats.
         log(stats, config);