I defined a unassigned user in my JIRA account and now I can assign to unassigned user. when I want to get all those tickets using quick filter - I can't. any solution / workaround will be most welcome.
In your case, create filter and type in JQL:
assignee = unassigned user
Or if no assignee
assignee is empty
You can select the widget "filter result" to show the consequence on your dashboard.