Ignore long-poll successes with no data

The server will occationally return successfully with no data during
a restart.  Previously, when we would get such responses, the page
would stop working because of JS errors.

(imported from commit 39b89907dc5ae9a9eb54ebf60d0069281e401786)

zephyr/static/js/zephyr.js

@@ -646,6 +646,14 @@ function get_updates() {
                            // minute to deal with crappy home wireless
                            // routers that kill "inactive" http connections.
         success: function (data) {
+            if (! data) {
+                // The server occationally returns no data during a
+                // restart.  Ignore those responses so the page keeps
+                // working
+                get_updates_timeout = setTimeout(get_updates, 0);
+                return;
+            }
+
             get_updates_params.failures = 0;
             $('#connection-error').hide();