Item-wise Sales Register Fails When Generating Report Without Customer Filter

### Information about bug

When generating the Item-wise Sales Register report without initially selecting a Customer filter, the report throws an `AttributeError: 'NoneType' object has no attribute 'customer_name'`. The report should display data for all customers when no customer is specified, but instead it fails.

Steps to Reproduce:

1. Open the Item-wise Sales Register report.
2. Set the required filters (Company, Date Range, etc.).
3. Leave the Customer filter blank to view data for all customers.
4. Generate the report.

### Module

accounts

### Version

ERPNext: v15.104.0 (version-15)
Frappe Framework: v15.99.0 (version-15)

### Installation method

None

### Relevant log output / Stack trace / Full Error Message.

```shell
row = {'item_code': 'TAFJW', 'item_name': 'FIBRE JOINT TAPE - 90Mtrs.', 'item_group': 'ACCESSORIES', 'description': 'FIBRE JOINT TAPE - 90Mtrs.', 'invoice': 'AJMCRI-2026-05495', 'posting_date': datetime.date(2026, 5, 29), 'customer': 'AL AMEEN INTERIOR DECORATIONS WORKS', 'customer_name': 'AL AMEEN INTERIOR DECORATIONS WORKS', 'customer_group': 'DEBTORS', 'debit_to': 'DEBTORS - NMRQ', 'mode_of_payment': '', 'territory': 'U.A.E', 'project': None, 'company': 'Namariq', 'sales_order': 'SAL-ORD-2026-14578', 'delivery_note': 'AJMDN-2026-06157', 'income_account': 'CREDIT SALES - NMRQ', 'cost_center': 'Ajman - NMRQ', 'stock_qty': 2.0, 'stock_uom': 'PCS', 'rate': 8.0, 'amount': 16.0, 'vat_5%_rate': 0, 'vat_5%_amount': 0, 'vat_output_account_rate': 5.0, 'vat_output_account_amount': 0.8, 'total_tax': 0.8, 'total_other_charges': 0, 'total': 16.8, 'currency': 'AED'}
total_tax = 0.8
total_other_charges = 0
item_tax = {'tax_rate': 5.0, 'tax_amount': 0.8, 'is_other_charges': 0}
builtins.AttributeError: 'NoneType' object has no attribute 'customer_name'
```