Don't close the current sitemap if we never opened one

dspace-api/src/main/java/org/dspace/app/sitemap/AbstractGenerator.java

@@ -151,8 +151,11 @@ public abstract class AbstractGenerator
      *             if an error occurs writing
      */
     public int finish() throws IOException
+    {
+        if (null != currentOutput)
         {
             closeCurrentFile();
+        }
 
         OutputStream fo = new FileOutputStream(new File(outputDir,
                 getIndexFilename()));