UserTask with blank userId and userGroup

[eduwercamacaro]

By definition user tasks can be either assigned to a userId or owned by a userGroup, but there are some cases where both userId and userGroup are null. Consider this example in:

return new WorkflowImpl(
            "example-ut",
            wf -> {
                WfRunVariable userId = wf.addVariable("userId", VariableType.STR);
                WfRunVariable userGroup = wf.addVariable("userGroup", VariableType.STR);
                wf.assignUserTask("my-task", userId, userGroup);
                wf.execute("my-task");
            }
        );

If you run this example without variables, the UserTaskRun will have null in both userId and userGroup.
I believe this can be solved by failing the UserTaskRun on arrival if the assignment is not valid.

[eduwercamacaro]

#1333 proposes a solution for this which fails with a VAR_ERROR if both userId and userGroup are null