$(document).ready(function() {
  $('.draft-btn').parent().hide();
  $('.polyType input').on('change', function () {
    if ($(this).val() == "") {
      $('.draft-btn').parent().hide();
    }
    else {
      $('.draft-btn').parent().show();      
    }
  })
});

Line 1: calls the function on page load (in your code, it checks your condition once on document ready and that's it)

Line 2: sets the Save as Draft button to hidden by default

Lines 5 + 8: hides or shows the button depending on whether the field is filled or not. Thing to note here is that the class you want to target is "draft-btn". You can always right click elements in your form preview page and click inspect to find the element's properties.