DatePicker: Fix crash when navigating between months

[mirka]

Description

A bug was introduced in #29716 that causes a onMonthPreviewed is not a function crash when the prev/next month buttons are clicked in DatePicker (but not DateTimePicker).

This PR:

• Adds a DatePicker story in Storybook for testing.
• Fixes the component so it doesn't call the onMonthPreviewed prop function if it wasn't passed.

How has this been tested?

Both the DatePicker and DateTimePicker components in Storybook work as expected.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[jorgefilipecosta]

I was able to replicate the bug using the storybook without the non-storybook change, and confirmed the change fixes the issue.