The value of a field can not be null, it's always a string value.

The code will check if the string value is the string "NULL". You want to check if it's an empty string instead:

if ($('#person_data[document_type]').val() != ''){}

or:

if ($('#person_data[document_type]').val().length != 0){}

If you want to check if the element exist at all, you should do that before calling val:

var $d = $('#person_data[document_type]');
if ($d.length != 0) {
  if ($d.val().length != 0 ) {...}
}