Corrected check for opening the dropdown

This corrects the check that was used when the dropdown was opened, so
the results would be immediately rendered instead of delaying the
loading. Previously it would delay the immediate load but immediately
load the results when the search term was emptied.

This closes select2#4191
This closes select2#4192
This closes select2#4202

Author: kevin-brown

src/js/select2/data/ajax.js

@@ -94,7 +94,7 @@ define([
       self._request = $request;
     }
 
-    if (this.ajaxOptions.delay && params.term !== '') {
+    if (this.ajaxOptions.delay && params.term != null) {
       if (this._queryTimeout) {
         window.clearTimeout(this._queryTimeout);
       }