chore(combo): Fixing aria-activedescendant test

Author: Ivan Kitanov

projects/igniteui-angular/src/lib/combo/combo.component.spec.ts

@@ -1073,7 +1073,7 @@ describe('igxCombo', () => {
 
                 const list = fixture.debugElement.query(By.css(`.${CSS_CLASS_CONTENT}`));
                 expect(list.nativeElement.getAttribute('aria-multiselectable')).toEqual('true');
-                expect(list.nativeElement.getAttribute('aria-activedescendant')).toEqual('');
+                expect(list.nativeElement.getAttribute('aria-activedescendant')).toEqual(null);
 
                 UIInteractions.triggerEventHandlerKeyDown('ArrowDown', list);
                 tick();