Two dimensional filter statistics gadget does not show empty values

Summary

Two dimensional filter statistics gadget's behaviour has changed since previous versions. If any issues have None value for some custom field, these tickets aren't counted in the gadget and there are no empty values (None) entries shown in the gadget (at least for multi-select-list custom fields).

Steps to Reproduce

1. Create custom field with multiple choices (eg. custom field named Browsers, can have internet browser as options - IE, Edge, Firefox, Chrome etc.)
2. Create a simple filter like "project = TEST and issuetype = Bug"
3. Add Two dimensional filter statistics gadget to your dashboard specifying the filter from Step 1
4. Set XAxis = Issue Type
5. Set YAxis = some multi-select-list custom fields, eg. Browser(s) field that has Internet browsers as options; Sort By, Sort Direction, Number of Results and Auto refresh settings - seems it does not matter what you put there
6. Save the filter.

Expected Results

Newly created gadget should show complete breakdown, including number that counts issues having None as a YAxis custom field's value (screenshot attached).

Actual Results

Newly created gadget doesn't include issues having None as a YAxis custom field's value. There is no row which displays None as a field value followed by issue count (screenshot attached).

Notes

The bug doesn't exist in versions 7.1.9 (tested by customer) and 7.3.8 (tested by Atlassian Support).
It exists in both 7.5.0 and 7.5.1 versions.

Workaround

No