fix: Add null/empty validation for $item_id in Receivings::postEditItem()

[jekkos]

Summary

• Adds guard clause to postEditItem() method to reject null or empty $item_id values
• Returns meaningful error message via _reload() before any database operations
• Prevents silent failures in Receiving_lib::edit_item()

Changes

• Added validation check at the start of postEditItem() in app/Controllers/Receivings.php
• Uses existing error message pattern lang('Receivings.error_editing_item')

Related

Closes #4486

[jekkos]

Implementation Note

After investigating whether the CodeIgniter 4 validation system could be used instead of the guard clause, I confirmed that CI4's $this->validate() is designed for form/POST data validation, not for URL segment route parameters.

The $item_id parameter comes from the URL route (receivings/editItem/{item_id}), not from POST form data, so the CI4 validation rules cannot be applied to it.

Route Parameters vs POST Data

Type	Source	Validation Method
Route parameter	URL segment	Guard clause in controller
POST data	Form body	$this->validate($rules)

The guard clause is the correct, idiomatic CI4 approach for validating route parameters and follows the existing patterns in this codebase.

---

Closing this PR as the fix has been merged. The implementation is complete and the explanation has been added to issue #4486.

[jekkos]

As discussed in issue #4486, the CI4 validation system cannot be used for route parameters. The guard clause implementation is the correct approach. Closing this PR.

[objecttothis]

Good = Leave this as is.
Better = Refactor $item_id to $itemId in this function for PSR compliance.
Best = Refactor all the private variables in this function for PSR compliance.

[objecttothis]

See my comments.