[FIX] hr_holidays: add fallback to user timezone

Version:
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Only present in V16.0.
Corrected in FW from saas-16.1
Introduced with this commit: fb774d98

Issue:
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If we have no employee in the context and
the user has no employee (in a multi-company context), we get an error.

Solution:
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Add a fallback on the user's timezone that we are sure is defined

opw-3381894

closes odoo/odoo#126915

Signed-off-by: Kevin Baptiste <kba@odoo.com>