If exportFileName hook isn’t defined don’t return empty filename.

This commit should fix #2251.

If ``exportFileName`` hook is not defined, ``hookFileName`` should be an empty array. Test the length of ``hookFileName`` before overriding ``fileName``, the export filename.

src/node/handler/ExportHandler.js

@@ -52,7 +52,7 @@ exports.doExport = function(req, res, padId, type)
   hooks.aCallFirst("exportFileName", padId, 
     function(err, hookFileName){
       // if fileName is set then set it to the padId, note that fileName is returned as an array.
-      if(hookFileName) fileName = hookFileName; 
+      if(hookFileName.length) fileName = hookFileName;
 
 
       //tell the browser that this is a downloadable file