fix: Make shaka.util.Lazy work with null (#8267)

Due to usage of `==` instead of `===` Lazy utility does not work
correctly with null generators, despite of what is stated in
documentation.

Author: tykus160

lib/util/lazy.js

@@ -28,11 +28,11 @@ shaka.util.Lazy = class {
 
   /** @return {T} */
   value() {
-    if (this.value_ == undefined) {
+    if (this.value_ === undefined) {
       // Compiler complains about unknown fields without this cast.
       this.value_ = /** @type {*} */ (this.gen_());
       goog.asserts.assert(
-          this.value_ != undefined, 'Unable to create lazy value');
+          this.value_ !== undefined, 'Unable to create lazy value');
     }
     return this.value_;
   }

test/util/lazy_unit.js

@@ -0,0 +1,33 @@
+/*! @license
+ * Shaka Player
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+describe('Lazy', () => {
+  it('returns the same object each time', () => {
+    const generator = jasmine.createSpy('generator').and.returnValue({});
+    const lazy = new shaka.util.Lazy(shaka.test.Util.spyFunc(generator));
+    const value = lazy.value();
+    const value2 = lazy.value();
+    expect(value).toBe(value2);
+    expect(generator).toHaveBeenCalledTimes(1);
+  });
+
+  it('works correctly for primitive value', () => {
+    const generator = jasmine.createSpy('generator').and.returnValue(7);
+    const lazy = new shaka.util.Lazy(shaka.test.Util.spyFunc(generator));
+    const value = lazy.value();
+    const value2 = lazy.value();
+    expect(value).toBe(value2);
+    expect(generator).toHaveBeenCalledTimes(1);
+  });
+
+  it('works correctly for null', () => {
+    const generator = jasmine.createSpy('generator').and.returnValue(null);
+    const lazy = new shaka.util.Lazy(shaka.test.Util.spyFunc(generator));
+    const value = lazy.value();
+    expect(value).toBe(null);
+    expect(generator).toHaveBeenCalledTimes(1);
+  });
+});