Task completion with empty payload overrides saved values

Issue

After upgrading from 6.3.0 to 6.4.4 a change in behavior has been observed for completing a task.
Having a task which maps a task output variable to a process instance variable.

Workflow looks like this:
- start task
- update the task output using kie-server/services/rest/server/containers//tasks/4/contents/output with json payload
- complete the task using {} as the payload