The JQL search doesn't return issues with empty values in the "Epic Link" field when using the clause "Epic Link" !='

Issue Summary

The JQL search doesn't return issues with empty values in the "Epic Link" field when using the clause "Epic Link" !='

Steps to Reproduce

1. Create a project and add some stories and epics to it
2. Link some stories to the epic
3. Get the issue key of the epic (let's say ABS-12) and go to the advanced search and type the search "Epic Link" != ABS-12

Expected Results

The search returns issues associated with other epics and stories with no epics

Actual Results

The search returns only issues associated with other epics

Workaround

Use the following filter:

"Epic Link" != ABS-12 or "Epic Link" is empty)