registerOption: Don't prevent adding empty string

Author: joallard

CHANGELOG.md

@@ -1,2 +1,3 @@
+* Fixed bug making `allowEmptyOption: true` useless (thanks @mcavalletto - #739)
 * Functions in option `render` can now return a DOM node in addition to
   text. (#617)

src/selectize.js

@@ -1184,7 +1184,7 @@ $.extend(Selectize.prototype, {
 	 */
 	registerOption: function(data) {
 		var key = hash_key(data[this.settings.valueField]);
-		if (!key || this.options.hasOwnProperty(key)) return false;
+		if (typeof key === 'undefined' || key === null || this.options.hasOwnProperty(key)) return false;
 		data.$order = data.$order || ++this.order;
 		this.options[key] = data;
 		return key;