🐛 [#4908] Fix bug in form variables list of configuration options

The configuration options could be saved without specifying any form variables to include. This does not make much sense. Added a minimum length of 1 for the form variables list in the serializer (Thanks to Robin for the hint)

src/openforms/registrations/contrib/json/config.py

@@ -11,8 +11,6 @@
 from openforms.utils.mixins import JsonSchemaSerializerMixin
 
 
-# TODO-4908: when you select a form variable in the configurations and then remove it again,
-#  it is possible to save the plugin and form without validation errors.
 class JSONOptionsSerializer(JsonSchemaSerializerMixin, serializers.Serializer):
     service = PrimaryKeyRelatedAsChoicesField(
         queryset=Service.objects.filter(api_type=APITypes.orc),
@@ -34,6 +32,7 @@ class JSONOptionsSerializer(JsonSchemaSerializerMixin, serializers.Serializer):
         label=_("Form variable key list"),
         help_text=_("A list of form variables (can also include static variables) to use."),
         required=True,
+        min_length=1,
     )