Environment (Required on creation):

Camunda Platform 7.16.0-ee

Description (Required on creation; please attach any relevant screenshots, stacktraces, log files, etc. to the ticket):

If you use the REST API to claim or set an assignee via POST:

task/{id}/claim | task/{id}/assignee

with an empty payload "{}" The result is that the task is unclaimed. This behavior should be documented for the REST API, as it is for the Java API, e.g.: TaskService.claim states:

userId - user that claims the task. When userId is null the task is unclaimed, assigned to no one.

Steps to reproduce (Required on creation):

1. Create a user task.
2. Claim the user task.
3. Try to claim or set an assignee via POST:

   task/{id}/claim | task/{id}/assignee

   with an empty payload "{}"