[FIX] purchase_requisition: adequate display of internal reference

To reproduce the issue:
1. Install Inventory & Purchase apps
2. Toggle on Settings > Inventory > Purchase Agreements and save
3. Go to Purchase
- [Products] > [Products]: add a product w/ internal reference and w/o
- [Orders] > [Blanket Orders]: create a blanket order with the products
- Save, Confirm and click Print icon to generate the said report in pdf

Desired behavior: Do not show brackets for null internal reference

opw-3138636

closes odoo/odoo#110812

X-original-commit: fb0567ee4fe75d3d0ab2475eaf1c7277ee24636a
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: Lee, Hansun (hale) <hale@odoo.com>

addons/purchase_requisition/report/report_purchaserequisition.xml

@@ -46,7 +46,9 @@
                             <tbody>
                                 <tr t-foreach="o.line_ids" t-as="line_ids">
                                     <td>
-                                        [ <span t-field="line_ids.product_id.code"/> ]
+                                        <t t-if="line_ids.product_id.code"><!--internal reference exists-->
+                                            [ <span t-field="line_ids.product_id.code"/> ]
+                                        </t>
                                         <span t-field="line_ids.product_id.name"/>
                                     </td>
                                     <td class="text-end">