fix(@angular-devkit/build-angular): build with empty assets array

Fix #10438 (comment)

Author: filipesilva

packages/angular_devkit/build_angular/src/utils/normalize-asset-patterns.ts

@@ -38,6 +38,13 @@ export function normalizeAssetPatterns(
   const sourceRoot = maybeSourceRoot || join(projectRoot, 'src');
   const resolvedSourceRoot = resolve(root, sourceRoot);
 
+  if (assetPatterns.length === 0) {
+    // If there are no asset patterns, return an empty array.
+    // It's important to do this because forkJoin with an empty array will immediately complete
+    // the observable.
+    return of([]);
+  }
+
   const assetPatternObjectObservables: Observable<AssetPatternObject>[] = assetPatterns
     .map(assetPattern => {
       // Normalize string asset patterns to objects.

packages/angular_devkit/build_angular/test/browser/assets_spec_large.ts

@@ -7,7 +7,7 @@
  */
 
 import { normalize, virtualFs } from '@angular-devkit/core';
-import { tap } from 'rxjs/operators';
+import { tap, toArray } from 'rxjs/operators';
 import { Timeout, browserTargetSpec, host, runTargetSpec } from '../utils';
 
 
@@ -91,4 +91,15 @@ describe('Browser Builder assets', () => {
     // node_modules folder will hit this one and can fail.
     host.scopedSync().delete(normalize('./node_modules'));
   }, Timeout.Basic);
+
+  it('still builds with empty asset array', (done) => {
+    const overrides = {
+      assets: [],
+    };
+
+    runTargetSpec(host, browserTargetSpec, overrides).pipe(
+      toArray(),
+      tap((buildEvents) => expect(buildEvents.length).toBe(1)),
+    ).subscribe(undefined, done.fail, done);
+  }, Timeout.Basic);
 });