Two dimensional filter statistics doesn't show the hyperlink for issues that are on Select List ( single choice ) of option "None" when the field is hidden

Summary

Two dimensional filter statistics doesn't show the hyperlink for issues that are on Select List ( single choice ) of option "None" when the field is hidden

Steps to Reproduce

1. Create a custom field of type Select List ( single choice )
   Eg: "Severity" with options "one", "two", "three"
2. On a sample project, assign random issues values for this field
3. Go to the dashboard and add Two dimensional filter statistics gadget.
4. On xaxis = Newly created custom field Eg: Severity
5. On yaxis = Issue Type
6. Now you should see the gadget and any issues that do not have a value for field "severity" will come under column None. These numbers under this column will have a hyperlink
7. Now go to the projects Field configuration and set the "severity" field to hidden
8. Come back to the gadget on the dashboard

Expected Results

You should still be able to get the hyperlink for issue that do not have a value for "severity" field

Actual Results

• The column "None" has now changed name to "Irrelevant" and the issues under them do not have a hyperlink as shown in the above screenshot.
• Update: In JIRA 7.5 the new bug was introduced (JRASERVER-66318) causing confusion due to the filter giving back a quantity of issues different than the count of issues considered in the gadget and this in turn make the numbers not matching at all, like in this example:
  
  It is important to note that those are two distinct issues. Please refer to JRASERVER-66318 if you are experiencing a missing "None" column.

Workaround

• None