Description:
------------
get_browser should work with null string and doesn't when using 5.3.1. 
5.2.11 works fine.

Reproduce code:
---------------
<?php
phpinfo();
echo "User Agent: " . $_SERVER['HTTP_USER_AGENT'] . "<br>";
echo 'print_r(get_browser($_SERVER[\'HTTP_USER_AGENT\'], true));<br>';
print_r(get_browser($_SERVER['HTTP_USER_AGENT'], true));
echo '<br>print_r(get_browser(null, true))<br>';
print_r(get_browser(null, true));
?>


Expected result:
----------------
That both get_browser calls should produce identical results.

Actual result:
--------------
The first call with $_SERVER['HTTP_USER_AGENT'] returns proper results 
and the call with null returns a default response (same as what it would 
return with a completely empty user agent string).