Flexible DateTime widget: Small UX improvement when clearing date

[stefan-korn]

User Story

The flexible DateTime widget (flexible_datetime) is setting the time part to "00:00:00" if a date is chosen but no time. This is convenient and avoids annoying validation error message.

It would also be nice, to have the time part cleared when you clear the date part via the HTML5 widget, but only when the time part is 00:00:00 probably. I would propose to have the time part cleared the same way the "00:00:00" time part is set (in valueCallback).

Acceptance Criteria

Clearing a date with time part "00:00:00" should allow to save and clear the time part while processing the value.