Do not attempt to parse blank responses from cancelled XHR POST requests

An XHR POST request may be cancelled by navigating away from the page while the timeline is loading.  This causes alert spam on Chrome and Safari.

core/src/main/resources/hudson/model/BuildTimelineWidget/control.jelly

@@ -59,10 +59,12 @@ THE SOFTWARE.
                   method:"POST",
                   parameters: {min: i*interval, max:(i+1)*interval},
                   onSuccess: function(t) {
-                    try {
-                      eventSource1.loadJSON(eval('('+t.responseText+')'),'.');
-                    } catch (e) {
-                      alert(e);
+                    if (t.status != 0) {
+                      try {
+                        eventSource1.loadJSON(eval('('+t.responseText+')'),'.');
+                      } catch (e) {
+                        alert(e);
+                      }
                     }
                   }
               });