Thanks Greg, you are right I can check the assignee variable if it is empty or not, but to perform a user task assignee is must.
In my scenario I need to start a process either by setting the assignee or without it, apart from that I need to maintain a process variable named 'status' as 'unassigned' in case when assignee is not provided which i can do in script task. I'm stuck at the point when no assignee is provided during start event and I want to move to 'check stock' User task only if assignee is provided. So let say if I don't provide the assignee during start event ,at inclusive gate i can check for the assignee field if it is not present I pass the flow to script task,At this point I have to hold on to a state until i get the assignee before moving to check stock. I don't find a way to get assignee at this point because no task is assigned to any user then how should i get the assignee field?