[#33923] PHP Notice: Undefined index: HTTP_USER_AGENT in (sysinfo.php + debug.php)

In Debug Plugin and in "System Information" view, it blindly tries to access the variable $_SERVER['HTTP_USER_AGENT'] without ensuring that it exists. This leads to 'PHP Notice: Undefined index: HTTP_USER_AGENT' when the browser does not send the USER AGENT string.

== How to reproduce the problem ==
1. Under Global Configuration, enable System > Debug System.
2. Anonymize your browser, by preventing to send the USER AGENT string. I used "HTTP UserAgent cleaner" (https://addons.mozilla.org/it/firefox/addon/http-useragent-cleaner/). Once installed on Firefox, you have to manually enable the anonymization of the User Agent string in the options: HTTP > User Agent. Ensure that this option is "Enabled", which corresponds to red color.
If you are unable to block USER AGENT string, simply establish a telnet connection as follows:

(The following telnet session supposes that your joomla runs on localhost)
telnet localhost 80
'When a prompt appear, type:'
GET /joomla/cms/ HTTP/1.0
'Hit enter key twice'

3. Enjoy the PHP Notices generated in the error log of the web server.

== Tracker ==
http://code.joomla.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33923&start=0