Fix notice in CakeSession

When first visiting a cake app, this notice happens:

    Notice (8): Undefined index: userAgent [APP\Vendor\cakephp\cakephp\lib\Cake\Model\Datasource\CakeSession.php, line 351]

This first checks on the existence.
But I wonder if there might be a more correct approach here - there must be reason the index is undefined.
Looks like in that case _checkValid() is invoked not soon enough.