fix(router): throw an error when navigate to null/undefined path

Closes #10560

Fixes #13384

modules/@angular/router/src/router.ts

@@ -526,6 +526,7 @@ export class Router {
    */
   navigate(commands: any[], extras: NavigationExtras = {skipLocationChange: false}):
       Promise<boolean> {
+    validateCommands(commands);
     if (typeof extras.queryParams === 'object' && extras.queryParams !== null) {
       extras.queryParams = this.removeEmptyProps(extras.queryParams);
     }
@@ -1238,3 +1239,12 @@ function getOutlet(outletMap: RouterOutletMap, route: ActivatedRoute): RouterOut
   }
   return outlet;
 }
+
+function validateCommands(commands: string[]): void {
+  for (let i = 0; i < commands.length; i++) {
+    const cmd = commands[i];
+    if (cmd == null) {
+      throw new Error(`The requested path contains ${cmd} segment at index ${i}`);
+    }
+  }
+}

modules/@angular/router/test/integration.spec.ts

@@ -536,6 +536,17 @@ describe('Integration', () => {
        expect(cmp.recordedParams).toEqual([{name: '1'}]);
      })));
 
+  it('should throw an error when one of the commands is null/undefined',
+     fakeAsync(inject([Router], (router: Router) => {
+       createRoot(router, RootCmp);
+
+       router.resetConfig([{path: 'query', component: EmptyQueryParamsCmp}]);
+
+       expect(() => router.navigate([
+         undefined, 'query'
+       ])).toThrowError(`The requested path contains undefined segment at index 0`);
+     })));
+
   it('should push params only when they change', fakeAsync(inject([Router], (router: Router) => {
        const fixture = createRoot(router, RootCmp);