FIX: /Info does not fail anymore.

See #83

src/main/java/caosdb/server/database/misc/TransactionBenchmark.java

@@ -236,7 +236,11 @@ class JdomConverter {
     Element ret = new Element(b.getName());
     ret.setAttribute("since", new Date(b.since).toString());
     ret.setAttribute("since_ms", Long.toString(b.since));
-    ret.setAttribute("called_from", b.called_from());
+    String calledFrom = b.called_from();
+    if (null == calledFrom) {
+      calledFrom = "-";
+    }
+    ret.setAttribute("called_from", calledFrom);
     Iterable<Measurement> measurements = b.getMeasurements();
 
     synchronized (measurements) {