fix(@schematics/angular): blank root leads to wrong tsconfig.lib.prod.json path

Closes: #16444

Author: alan-agius4

packages/schematics/angular/migrations/update-9/ivy-libraries.ts

@@ -5,6 +5,8 @@
  * Use of this source code is governed by an MIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
+
+import { join, normalize } from '@angular-devkit/core';
 import { Rule, Tree } from '@angular-devkit/schematics';
 import { getWorkspacePath } from '../../utility/config';
 import {
@@ -36,7 +38,7 @@ export function updateLibraries(): Rule {
       }
 
       const configurations = findPropertyInAstObject(target, 'configurations');
-      const tsConfig = `${projectRoot.value}/tsconfig.lib.prod.json`;
+      const tsConfig = join(normalize(projectRoot.value), 'tsconfig.lib.prod.json');
 
       if (!configurations || configurations.kind !== 'object') {
         // Configurations doesn't exist.

packages/schematics/angular/migrations/update-9/ivy-libraries_spec.ts

@@ -64,6 +64,20 @@ describe('Migration to version 9', () => {
       tree.delete(libProdTsConfig);
     });
 
+    it(`should add 'tsConfig' option with correct path when 'root' is an empty string`, async () => {
+      let config = JSON.parse(tree.readContent(workspacePath));
+      const projectConfig = config.projects['migration-lib'];
+      projectConfig.architect.build.configurations = undefined;
+      projectConfig.root = '';
+      tree.overwrite(workspacePath, JSON.stringify(config, undefined, 2));
+
+      const libProdTsConfigPath = 'tsconfig.lib.prod.json';
+      const tree2 = await schematicRunner.runSchematicAsync('workspace-version-9', {}, tree.branch()).toPromise();
+      config = getWorkspaceTargets(tree2).build;
+      expect(config.configurations.production.tsConfig).toEqual(libProdTsConfigPath);
+      expect(tree2.exists(libProdTsConfigPath)).toBeTruthy();
+    });
+
     it(`should add 'tsConfig' option in production when configurations doesn't exists`, async () => {
       let config = getWorkspaceTargets(tree);
       config.build.configurations = undefined;