[FIX] base: barcode fallback

- Set a product barcode to a 8 letters value, e.g. 'AAAAAAAA'
- Print 'Product Barcode (PDF)' (or any other report containing the
  barcode)

The barcode doesn't print.

All templates assume that:
- a 13 characters barcode is EAN13
- a 8 characters barcode is EAN8

But this assumption is too restrictive: one could want to generate
barcodes with such length without following the EAN13 or EAN8 structure.

We fall back on Code128 when the barcode generation fails.

opw-1937858

closes odoo/odoo#30912

odoo/addons/base/models/ir_actions_report.py

@@ -467,7 +467,10 @@ class IrActionsReport(models.Model):
             )
             return barcode.asString('png')
         except (ValueError, AttributeError):
-            raise ValueError("Cannot convert into barcode.")
+            if barcode_type == 'Code128':
+                raise ValueError("Cannot convert into barcode.")
+            else:
+                return self.barcode('Code128', value, width=width, height=height, humanreadable=humanreadable)
 
     @api.multi
     def render_template(self, template, values=None):