fix: fix to fetch customers and billing email in PSOA (backport #27363)

[mergify]

Issue:
Process Statement of Accounts -

• System throws the following error even when one has a customer with primary contact email because billing email is missing.
  
• Query to fetch billing email failed to do so

Fix:

• Fixed the condition to fetch customers
• Fixed a query to fetch billing email
• Added a description to the checkbox which goes otherwise unnoticed
  

---

This is an automatic backport of pull request #27363 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a58e309 has failed:

On branch mergify/bp/version-13-hotfix/pr-27363
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	both modified:   erpnext/accounts/doctype/process_statement_of_accounts/process_statement_of_accounts.json
	both modified:   erpnext/accounts/doctype/process_statement_of_accounts/process_statement_of_accounts.py

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