try this:
assuming your datatable is dt
dt=(dt.AsEnumerable().Select(delegate(DataRow r)
{
DataRow nr = dt.NewRow();
foreach (DataColumn item in dt.Columns)
{
nr.SetField(item.ColumnName, r[item.ColumnName].ToString() =="0" ? null : r[item.ColumnName]);
}
return nr;
})).CopyToDataTable();