Allow not to ask for invoice addresses on free orders

src/pretix/control/forms/item.py

@@ -43,6 +43,7 @@ class QuestionForm(I18nModelForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.fields['items'].queryset = self.instance.event.items.all()
+        self.fields['items'].required = True
         self.fields['dependency_question'].queryset = self.instance.event.questions.filter(
             type__in=(Question.TYPE_BOOLEAN, Question.TYPE_CHOICE, Question.TYPE_CHOICE_MULTIPLE)
         )