DateTime: Add default date/time to stories

[tyxla]

What?

Fixes a couple of warnings in the DateTime component family stories.

Why?

There are warnings because we don't have an initial date or time.

Found while working on #67574.

How?

We're updating the stories to have initial date or time set.

Testing Instructions

• Open the Time & Date stories and verify the warning no longer appears:
  • DateTimePicker: http://localhost:50240/?path=/docs/components-datetimepicker--docs
  • DatePicker: http://localhost:50240/?path=/docs/components-datepicker--docs
  • TimePicker: http://localhost:50240/?path=/docs/components-timepicker--docs

Testing Instructions for Keyboard

Same

Screenshots or screencast

Warnings before this PR:

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Co-authored-by: Mamaduka <[email protected]>

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[mirka]

I think this means we have a bug? The TS types say currentDate can be undefined, and the code looks like it wants to handle currentDate being falsey.

I'm also noticing some discrepancy between the prop description ("Optionally pass in a null value to specify no date is currently selected") and the actual behavior (currentDate={ undefined } and currentDate={ null } both fall back to new Date() being selected).