fix(common): don't convert null to a string when flushing a mock request (#21417)

A bug in TestRequest caused null response bodies to be stringified. This
change causes null to be treated faithfully.

Fixes #20744

PR Close #21417

packages/common/http/testing/test/request_spec.ts

@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * 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 {ddescribe, describe, iit, it} from '@angular/core/testing/src/testing_internal';
+
+import {HttpClient} from '../../src/client';
+import {HttpClientTestingBackend} from '../src/backend';
+
+describe('HttpClient TestRequest', () => {
+  it('accepts a null body', () => {
+    const mock = new HttpClientTestingBackend();
+    const client = new HttpClient(mock);
+
+    let resp: any;
+    client.post('/some-url', {test: 'test'}).subscribe(body => { resp = body; });
+
+    const req = mock.expectOne('/some-url');
+    req.flush(null);
+
+    expect(resp).toBeNull();
+  });
+});