fix(report_utils): ensure that delimiter and separator can't be empty (backport #26165)

[mergify]

This results in them setting as undefined, which ends up as a string
in python, and the delimiter ends up as 'u', and the separator as
'undefined'.

---

This is an automatic backport of pull request #26165 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of af611d4 has failed:

On branch mergify/bp/version-14-hotfix/pr-26165
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	both modified:   frappe/public/js/frappe/views/reports/report_utils.js

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[frappe-pr-bot]

🎉 This PR is included in version 14.73.0 🎉

The release is available on GitHub release

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