fix: using DN for transfer w/o internal customer (backport #27798) (backport #27805)

[mergify]

This is an automatic backport of pull request #27805 done by Mergify.
Cherry-pick of fa94438 has failed:

On branch mergify/bp/develop/pr-27805
Your branch is up to date with 'origin/develop'.

You are currently cherry-picking commit fa944382c5.
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Changes to be committed:
	modified:   erpnext/controllers/selling_controller.py
	modified:   erpnext/stock/doctype/delivery_note/delivery_note.py
	modified:   erpnext/stock/doctype/delivery_note_item/delivery_note_item.json

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py

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

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