Fix storage realm username GroupBy

[jtpalmer]

Description

Fixes the storage realm GroupByUsername class.

Motivation and Context

This bug breaks the system username filter.

Tests performed

Tested manually.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.