check for existence of dataTransfer

As done in the other place where `dataTransfer` is used.
This is to support fake html5 drag and drop events, where `dataTransfer` is not accessible. Use case: Selenium testing with ePages-de/chromedriver-html5-dragdrop

Sortable.js

@@ -955,7 +955,9 @@
 
 
 	function _globalDragOver(/**Event*/evt) {
+		if (evt.dataTransfer) {
 			evt.dataTransfer.dropEffect = 'move';
+		}
 		evt.preventDefault();
 	}