[FIX] hr_attendance: Fix attendance creation if no default user

Steps to reproduce:
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- Create user with default company A
- User may have related employees, but not in company A
- Go to attendance - click -> New
You will get a traceback related to the sql request of _compute_overtime_hours

Explanation:
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- The query needs a non-empty list of employees

Fix:
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- Don't run the sql query if we haven't defined any employees yet on the attendance

closes odoo/odoo#141808

Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>