barcode_parser_guard

Odoo's barcode parser reads barcode.length without checking a barcode was passed. Callers can reach it with undefined, the JavaScript throws, and the scanner screen stops responding — operator at a cart, gun in hand, no way forward but a page reload.

• ✓ Guards the falsy case, defers to core for every valid scan
• ✓ Standard patch() helper — no core file touched
• ✓ Installs automatically alongside barcodes