Do not copy the template code

When the variant has no code, the template code is the only part of the unique
constraint. In such case copying the template fails because the new variant
will have the same code.

issue9462
review306081002

product.py

@@ -217,6 +217,15 @@ class Template(
         products = sum((t.products for t in templates), ())
         Product.sync_code(products)
 
+    @classmethod
+    def copy(cls, templates, default=None):
+        if default is None:
+            default = {}
+        else:
+            default = default.copy()
+        default.setdefault('code', None)
+        return super().copy(templates, default=default)
+
     @classmethod
     def search_global(cls, text):
         for record, rec_name, icon in super(Template, cls).search_global(text):