🐛 [#4765] Fixed bug in fix_empty_default_value migration

In case the `multiple` is true, but the `defaultValue` is set to None, the migration tried to iterable over None value

src/openforms/formio/migration_converters.py

@@ -280,6 +280,10 @@ def fix_empty_default_value(component: Component) -> bool:
  changed = False
 
  if component.get("multiple", False):
+ if default_value is None:
+ component["defaultValue"] = []
+ return True
+
  for index, value in enumerate(default_value):
  if value is None:
  component["defaultValue"][index] = ""