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
I agree. But my goal is not to indicate that the user agent has not been set, but simply set the correct data type to string.
Other pieces of code around there use string functions. They probably don't expect that $this->info['useragent'] is a null value and further warnings would be raised.