fix(router): add null support for RouterLink directive (#32616)

Value of "undefined" passed as segment in routerLink is stringified to string "undefined".
This change introduces the same behavior for value of "null".

PR Close #32616

packages/router/test/create_url_tree.spec.ts

@@ -68,7 +68,21 @@ describe('createUrlTree', () => {
     expect(params[1].path).toEqual('11');
   });
 
-  it('should support first segments contaings slashes', () => {
+  it('should work if command = null', () => {
+    const p = serializer.parse('/a/b');
+    const t = createRoot(p, [null]);
+    const params = t.root.children[PRIMARY_OUTLET].segments;
+    expect(params[0].path).toEqual('null');
+  });
+
+  it('should work if command is undefined', () => {
+    const p = serializer.parse('/a/b');
+    const t = createRoot(p, [undefined]);
+    const params = t.root.children[PRIMARY_OUTLET].segments;
+    expect(params[0].path).toEqual('undefined');
+  });
+
+  it('should support first segments containing slashes', () => {
     const p = serializer.parse('/');
     const t = createRoot(p, [{segmentPath: '/one'}, 'two/three']);
     expect(serializer.serialize(t)).toEqual('/%2Fone/two%2Fthree');

packages/router/test/integration.spec.ts

@@ -1994,6 +1994,36 @@ describe('Integration', () => {
          expect(() => buttons[1].click()).not.toThrow();
        }));
 
+    it('should not throw when some command is null', fakeAsync(() => {
+         @Component({
+           selector: 'someCmp',
+           template:
+               `<router-outlet></router-outlet><a [routerLink]="[null]">Link</a><button [routerLink]="[null]">Button</button>`
+         })
+         class CmpWithLink {
+         }
+
+         TestBed.configureTestingModule({declarations: [CmpWithLink]});
+         const router: Router = TestBed.inject(Router);
+
+         expect(() => createRoot(router, CmpWithLink)).not.toThrow();
+       }));
+
+    it('should not throw when some command is undefined', fakeAsync(() => {
+         @Component({
+           selector: 'someCmp',
+           template:
+               `<router-outlet></router-outlet><a [routerLink]="[undefined]">Link</a><button [routerLink]="[undefined]">Button</button>`
+         })
+         class CmpWithLink {
+         }
+
+         TestBed.configureTestingModule({declarations: [CmpWithLink]});
+         const router: Router = TestBed.inject(Router);
+
+         expect(() => createRoot(router, CmpWithLink)).not.toThrow();
+       }));
+
     it('should update hrefs when query params or fragment change', fakeAsync(() => {
          @Component({
            selector: 'someRoot',