Fix test failures caused by Bug 762787

Bugzilla/Constants.pm

@@ -569,7 +569,7 @@ use constant PASSWORD_SALT_LENGTH => 8;
 # via POST such as buglist.cgi. This value determines whether the redirect
 # can be safely done or not based on the web server's URI length setting.
 # See http://support.microsoft.com/kb/208427 for why MSIE is different
-use constant CGI_URI_LIMIT => $ENV{'HTTP_USER_AGENT'} =~ /MSIE/ ? 2083 : 8000;
+use constant CGI_URI_LIMIT => ($ENV{'HTTP_USER_AGENT'} || '') =~ /MSIE/ ? 2083 : 8000;
 
 # If the user isn't allowed to change a field, we must tell him who can.
 # We store the required permission set into the $PrivilegesRequired