Transactional callback isn't returning false

Trying to fix any situation that if we've something callback with explicit FALSE return, like:
```php
$rtn = $mgr->transactional(function() {...; return false;});
$rtn //true 
```
Should be the return value of the callback method, 
If the method is not returning any value (null), then.. we consider the callback was done, then return true.

Regards.