[#15] use Autowired(required=false) instead of Inject

 * the DummyNoOpExceptionFitler was there just to make Spring happy
   when using @Inject. Better solution is to use @Autowired(required=false)
   and remove the DummyNoOpExceptionFilter altogetheir (thanks @thradec
   for pointing this out)
 * added test that makes sure that the validator can handle the state
   where no filters are defined (e.g. the injected instance is null)