默认值被空值覆盖

I have got this simple insert in to database, but unfortunately default value inside DB which is N/A is getting overwrite by empty value if nothing is inserted in to field. Below is the code for insert, how can I make it to stay N/A?

$insertSQL = sprintf("INSERT INTO application_forms (application_type) VALUES (%s)", GetSQLValueString($_POST['application_type'], "text"));

Test for it being empty and if so don't insert.