IgbDataGrid empty filter only filters on string.Empty and not null for string columns

[agoldenbaum]

Steps to Reproduce

1. Run the attached sample.
2. Apply the "Empty" filter operator to the "Product" column.

Result
The string.empty cell value remains, but the null one is filtered out.

Expected Result
string.empty and null should remain, as the cell value is empty.

binding-local-data.zip