[FEAT] Update product_class str method (#4248)

* feat ⭐ update product_class str method

* fix 🔧 update github actions

* Revert "fix 🔧 update github actions"

This reverts commit 604a0ce.

src/oscar/apps/catalogue/abstract_models.py

@@ -115,7 +115,7 @@ class Meta:
         verbose_name_plural = _("Product classes")
 
     def __str__(self):
-        return self.name
+        return self.name or self.slug
 
     @property
     def has_attributes(self):