test: add coverage for outsideClickIgnoreClass undefined fallback

Adds test case to cover the scenario where outsideClickIgnoreClass is
explicitly set to undefined, ensuring the fallback to default props
is properly tested and improves test coverage for line 1327 in index.tsx.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: martijnrusschen

src/test/datepicker_test.test.tsx

@@ -711,6 +711,15 @@ describe("DatePicker", () => {
     expect(input?.classList.contains(outsideClickIgnoreClass)).toBe(true);
   });
 
+  it("should apply the default outsideClickIgnoreClass when prop is undefined", () => {
+    const { container } = render(
+      <DatePicker outsideClickIgnoreClass={undefined} />,
+    );
+    const input = safeQuerySelector<HTMLInputElement>(container, "input");
+    fireEvent.focus(input);
+    expect(input?.classList.contains(OUTSIDE_CLICK_IGNORE_CLASS)).toBe(true);
+  });
+
   it("should toggle the open status of calendar on click of the icon when toggleCalendarOnIconClick is set to true", () => {
     const { container } = render(
       <DatePicker