Fix 'undefined is not an object' traceback with update_active_status.

This seems to only happen with Firefox for some reason.  We've had
similar issues with other ajax endpoints, so this is probably not the
long-term solution, but I'd like to get this traceback fixed now.

(imported from commit aa552fb56882ae2c73e352c7baf9532a88c5cf0a)

zephyr/static/js/activity.js

@@ -96,6 +96,13 @@ function focus_ping() {
     }
 
     $.post('/json/update_active_status', {status: ACTIVE}, function (data) {
+        if (data === undefined || data.presences === undefined) {
+            // We sometimes receive no data even on successful
+            // requests; we should figure out why but this will
+            // prevent us from throwing errors until then
+            return;
+        }
+
         user_info = {};
 
         // Update Zephyr mirror activity warning