NULL values shows as 'None' in the edit mode

[theoziran]

When we click in a table to edit a column value containing 'NULL' it shows as 'None' and if you 'Save' it is replacing 'NULL' to 'None' which is unexpected behavior.
One suggestion to fix this issue is to add a checkbox to set the content of the column as NULL, then it would be easier to differentiate from empty to NULL.