Unify inconsistent notifyListener() logic

Previously, this function required a `queueEmptied` bool, which I changed
to `queueRunning`.  But I goofed the logic on this bool; it should
always be "is `processing` nullptr."

rtgui/batchqueue.h

@@ -93,7 +93,7 @@ protected:
     Glib::ustring autoCompleteFileName (const Glib::ustring& fileName, const Glib::ustring& format);
     Glib::ustring getTempFilenameForParams( const Glib::ustring &filename );
     bool saveBatchQueue ();
-    void notifyListener (bool queueRunning);
+    void notifyListener ();
 
     BatchQueueEntry* processing;  // holds the currently processed image
     FileCatalog* fileCatalog;