All the structural steps should be automatic nodes instead of tasks

When building the workflow we can use a "structural" node ( fork 2 ways, fork 3 ways). These should be automatic nodes instead of tasks nodes, because their role it's only to offer some predefined paths in the workflow execution.
Currently since they are tasks nodes, if we don't specify any tasks assignees on them, the workflow execution is blocked when reaching any of these in the graph execution.