sp_fr: do not create QName with None namespace (#31595)

It's not supported with lxml<4.3.3.

passerelle/apps/sp_fr/xsd.py

@@ -116,7 +116,7 @@ class Schema(object):
                 target = self.elements
             else:
                 raise NotImplementedError
-            if not d['name'].namespace:
+            if not d['name'].namespace and self.target_namespace:
                 d['name'] = ET.QName(self.target_namespace, d['name'].localname)
             target[d['name']] = d