FLEX-32541
CAUSE:
AdvancedDataGridHeaderRenderer.mouseEventToHeaderPart() assumes that sortItemRendererInstance cannot be null. However, as the project attached to the ticket shows, it can.

SOLUTION:
When sortItemRendererInstance is null, we return AdvancedDataGrid.HEADER_TEXT_PART, as there is no icon.
FLEX-32541 Removing unused constant.