Bug 38027: Set date to empty value instead of using flatpickr clear()

Bug 38027: Set date to empty value instead of using flatpickr clear()

1. Find a datetime field in Koha. For example, specifying due dates on the checkout page.
2. Specify a datetime.
3. Use the red 'X' icon to clear the date.
4. Notice a console error 'parsedDate' is undef
5. Now the flatpickr is date, not datetime
6. APPLY patch
7. Try again, this time no console error and the flatpickr remains a datetime one

Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>