jQuery: Check if the value of a field is empty (null)

The value of the field cannot be null, it is always a string value.

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

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

either:

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

If you want to check if the element exists, you should do the following before calling val:

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