[JENKINS-21292] Do not attempt to parse blank responses from cancelle…

…d 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.
(cherry picked from commit 72bb477)

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);
+                      }
                     }
                   }
               });