I want to delete 2 (custom) attributes defined via client representation in body parameter "attributes" via REST admin command "Update the client" (PUT method).
1.) customAttr_1 via not defining in JSON's request body
2.) customAttr_2 via setting with value empty string ""

Execution of Update The Client REST request performs successfully with status code 204 No Content.

Next I want to check the client representation via GET method.
The response returns customAttr_1 and does not return customAttr_2.
But If I want to re-create the deleted customAttr_2 (via UPDATE the client), I receive HTTP status code 409 Conflict.
Conclusion:
1.) customAttr_1 not deleted
2.) customAttr_2 not present in GET response, but still present in DB in table CLIENT_ATTRIBUTES with value Null.