fix: handle None values in earnings and deductions to avoid TypeError (backport #2715)

[mergify]

Update calculations of payable amount to ensure None values in earnings or deductions default to 0. This prevents TypeError.

---

This is an automatic backport of pull request #2715 done by Mergify.

[mergify]

Cherry-pick of 8f5581f has failed:

On branch mergify/bp/version-14-hotfix/pr-2715
Your branch is up to date with 'origin/version-14-hotfix'.

You are currently cherry-picking commit 8f5581f7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   hrms/payroll/doctype/payroll_entry/payroll_entry.py

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally