In the Issue Navigator, if you filter on "not equal to <field>" then the search query returns only tickets that have an field value that is not equal to the specified in the filter, but does not return issues where that field is empty.

Steps to reproduce:
1. go to the Issue Navigator and enter a search query, e.g. Project = <project_name>

2. change the query to exclude a value in on of the fields, here Project = <project_name> and Sprint != <sprint_ID>

Result: Issues not assigned to any Sprint aren't included in the results.