Issues with Hidden Field are not Sorting Properly

Steps to Reproduce

1. Create Project A with Default Field Configuration
2. Create Project B with 'Due Date' as a hidden field
3. Create 6 Issues in Project A, 3 with Due Dates fields, 3 Without
4. Create 3 Issues in Project B
5. Go to Issue Navigator and Sort by Due Date

Expected Results

Issues with Empty Due Date field are grouped together, and issues with Due Date values are grouped

Actual results

Issues with Hidden Due Date grouped, Issues with Due Date value are grouped, and Issues with empty Due Date values are grouped

Hidden fields should be treated the same as fields with no values.