Use None as default value for module of view

Since 53fc70c2139f the module column of ir_ui_view has a foreign key constraint
on ir_module table so the default value must be NULL to ignore the constraint
when there is no module.

Closes #13816

trytond/trytond/ir/ui/view.py

@@ -129,7 +129,7 @@ class View(
 
     @staticmethod
     def default_module():
-        return Transaction().context.get('module') or ''
+        return Transaction().context.get('module')
 
     def get_basis(self, name):
         return not self.inherit or self.model != self.inherit.model