[JENKINS-23774] Better report the case that the incoming web request has no Referer header set.

core/src/main/java/hudson/diagnosis/ReverseProxySetupMonitor.java

@@ -64,7 +64,7 @@ public class ReverseProxySetupMonitor extends AdministrativeMonitor {
         Jenkins j = Jenkins.getInstance();
         assert j != null;
         // May need to send an absolute URL, since handling of HttpRedirect with a relative URL does not currently honor X-Forwarded-Proto/Port at all.
-        String redirect = j.getRootUrl() + "administrativeMonitor/" + id + "/testForReverseProxySetup/" + Util.rawEncode(referer) + "/";
+        String redirect = j.getRootUrl() + "administrativeMonitor/" + id + "/testForReverseProxySetup/" + (referer != null ? Util.rawEncode(referer) : "NO-REFERER") + "/";
         LOGGER.log(Level.FINE, "coming from {0} and redirecting to {1}", new Object[] {referer, redirect});
         return new HttpRedirect(redirect);
     }