fix: correct salary slip naming when created via data import (backport #4980)

[mergify]

What's the problem?

When a Salary Slip is created via Data Import, the name comes out wrong — for example Sal Slip/None/00001 instead of Sal Slip/HR-EMP-00010/00001.

Why does it happen?

default_series was set as an instance variable in __init__ using self.employee. At that point, Frappe hasn't set any field values yet, so self.employee is None.

Fix

Changed default_series from an instance variable to a @property. It now reads self.employee lazily — at the time autoname() actually calls it, the employee field is already set.

Test

1. Create a Data Import for Salary Slip with an employee and period.
2. Run the import.
3. Verify the generated name follows the pattern Sal Slip/<employee>/<number>.

Before:

Screen.Recording.2026-07-21.at.4.57.25.PM.mov

After:

Screen.Recording.2026-07-21.at.5.13.32.PM.mov

no-docs

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This is an automatic backport of pull request #4980 done by [Mergify](https://mergify.com).

[mergify]

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	both modified:   hrms/payroll/doctype/salary_slip/salary_slip.py

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