ensured EntitySelector can cope with optional attribute

previously `data-entities="[]"` needed to be used when there was no
existing selection

app/assets/javascripts/iqvoc/entityselect.js.erb

@@ -26,7 +26,8 @@ var EntitySelector = function(node) {
 
  this.indicator.css("visibility", "hidden");
 
- var selection = $.map(this.el.data("entities"), function(entity, i) {
+ var selection = this.el.data("entities") || [];
+ selection = $.map(selection, function(entity, i) {
  return self.createEntity(entity);
  });
  selection = $('<ul class="entity_list" />').append(selection);