[FIX] product_matrix : templates with one attribute line.

Don't show "Not Available" as Line header when there is only one
attribute line on the product.template.

closes odoo/odoo#35586

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>

addons/product_matrix/models/product_template.py

@@ -75,8 +75,8 @@ class ProductTemplateAttributeValue(models.Model):
         :rtype: dict
         """
         header_cell = {
-            'name': '-'.join([attr.name for attr in self])
-        }
+            'name': '-'.join([attr.name for attr in self]) if self else " "
+        }  # The " " is to avoid having 'Not available' if the template has only one attribute line.
         extra_price = sum(self.mapped('price_extra')) if display_extra else 0
         if extra_price:
             sign = '+ ' if self.price_extra > 0 else '- '