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 https://github.com/select2/select2/issues/4191
This closes https://github.com/select2/select2/pull/4192
This closes https://github.com/select2/select2/pull/4202

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);
       }