How to preserve a value of NULL as on update as it is being set to 0

You can set the field to appear in the grid, but NOT on the edit form. Use set_col_property() to set editable to false and hidedlg to true. 

$dg->set_col_property('COLUMN_NAME', array('editable'=>false,'hidedlg'=>true));

Check out the 'tips' section in column hidden for more details.

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