Adding EntityType as extend of another EntityType via MetaDataApi does not work

[dtroelofsprins]

How to Reproduce

https://molgenis.gitbook.io/molgenis/interoperability/guide-api-metadata, try to create a new entity type with extend (post metadata)
with something like this:
{ "id": "eu_bbmri_eric_test", "package": "eu_bbmri_eric", "label": { "defaultValue": "Test Table", "translations": {} }, "description": { "defaultValue": "Test extend table", "translations": {} }, "extends": "eu_bbmri_eric_ontology_terms" }

Expected behavior

The table is created

Observed behavior

Error: "attributes must not be empty"

When including "attributes" : [] or something like that, it also results in an error