Fix error on Base Model `get_object_vars()`

Fixes errors.
`[08-May-2014 19:09:53] get_object_vars() expects parameter 1 to be object, null given in /data/webapps/bbc.co.uk/tviplayer/webapp/php/lib/vendor/bbc/bamboo2/src/Bamboo/Models/Base.php on line 12 while serving /tv/bbcalba`

Add check for object.