🐛 [#4796] Default value on non-required serializer fields in ZGW API …

…registration raises errors

The default value of ZGW zaaktype and case_type_identification raise errors when importing. The logic should be that one of the two is required, and creating a form using that logic works. But when you export and import a form with the ZGW API registration, that only has 'zaaktype', the import gives a "case_type_identification is required" error message

src/openforms/registrations/contrib/zgw_apis/options.py

@@ -70,7 +70,6 @@ class ZaakOptionsSerializer(JsonSchemaSerializerMixin, serializers.Serializer):
             "will automatically be selected based on the submission completion "
             "timestamp. When you specify this field, you MUST also specify a catalogue."
         ),
-        default="",
     )
     product = serializers.CharField(
         required=False,
@@ -85,7 +84,6 @@ class ZaakOptionsSerializer(JsonSchemaSerializerMixin, serializers.Serializer):
     zaaktype = serializers.URLField(
         required=False,
         help_text=_("URL of the ZAAKTYPE in the Catalogi API"),
-        default="",
     )
     informatieobjecttype = serializers.URLField(
         required=True,