fix(datebox): isStub && Update test

Author: marker dao ®

packages/devextreme/js/__internal/ui/date_box/m_date_box.mask.ts

@@ -476,7 +476,7 @@ class DateBoxMask extends DateBoxBase {
     }
     // @ts-expect-error ts-error
     let index = fitIntoRange(this._activePartIndex + step, 0, this._dateParts.length - 1);
-    if (this._dateParts[index].isStub) {
+    if (this._dateParts[index]?.isStub) {
       const isBoundaryIndex = index === 0 && step < 0 || index === this._dateParts.length - 1 && step > 0;
       if (!isBoundaryIndex) {
         this._selectNextPart(step >= 0 ? step + 1 : step - 1);

packages/devextreme/testing/tests/DevExpress.ui.widgets.editors/datebox.mask.tests.js

@@ -1302,7 +1302,8 @@ module('Date AM/PM Handling', setupModule, () => {
 
             this.instance.option({
                 value: new Date('10/10/2012 22:00'),
-                displayFormat: 'a'
+                displayFormat: 'a',
+                useMaskBehavior: true,
             });
 
             assert.strictEqual(this.$input.val(), pmName, 'initial value is localized PM');