feat(cdk/menu): Allow setting cdkMenuTrigger null

Add ability to set [cdkMenuTrigger]="null" as is now possible with material menu.

Fixes #25782

Author: klinki

src/cdk/menu/menu-trigger-base.ts

@@ -58,7 +58,7 @@ export abstract class CdkMenuTriggerBase implements OnDestroy {
   readonly closed: EventEmitter<void> = new EventEmitter();
 
   /** Template reference variable to the menu this trigger opens */
-  menuTemplateRef: TemplateRef<unknown>;
+  menuTemplateRef: TemplateRef<unknown> | null;
 
   /** Context data to be passed along to the menu template */
   menuData: unknown;

src/cdk/menu/menu-trigger.ts

@@ -44,7 +44,7 @@ import {CdkMenuTriggerBase, MENU_TRIGGER} from './menu-trigger-base';
   exportAs: 'cdkMenuTriggerFor',
   host: {
     'class': 'cdk-menu-trigger',
-    'aria-haspopup': 'menu',
+    '[attr.aria-haspopup]': 'menuTemplateRef ? "menu" : null',
     '[attr.aria-expanded]': 'isOpen()',
     '(focusin)': '_setHasFocus(true)',
     '(focusout)': '_setHasFocus(false)',
@@ -98,7 +98,7 @@ export class CdkMenuTrigger extends CdkMenuTriggerBase implements OnDestroy {
 
   /** Open the attached menu. */
   open() {
-    if (!this.isOpen()) {
+    if (!this.isOpen() && this.menuTemplateRef != null) {
       this.opened.next();
 
       this.overlayRef = this.overlayRef || this._overlay.create(this._getOverlayConfig());