[FIX] hr_holidays: no date_from creates error

Probem:
With some time off type as default type in an allocation request
you get a traceback as soon as you try to open the allocation
request form.

Cause:
FIX commit [1] introduced the issue as it didn't consider the
possibility of a false date_from

Improvement 1:
Having a date_from False sets a False in the leave request name
displayed on the form, we change that to have an empty string
instead

Improvement 2:
The timezone bug fix in commit [1] is only applied on a specific
configuration, we applie it to all configuration of leave request
name

[1] 5a50dea2652cff014d9198f59265cf36a3ec1865

opw-2961344

closes odoo/odoo#99454

X-original-commit: 3b9a012e4a0839bc5da2daf51d56591d2863b4be
Signed-off-by: Kevin Baptiste <kba@odoo.com>
Signed-off-by: Fockedey Martin (mafo) <mafo@odoo.com>