Fix entityvalidate property condition

This fix the PHP Notice of:
`Notice: Undefined index: property in RestfulEntityBase->entityValidate() (line 1400 of /path/to/project/modules/contrib/restful/plugins/restful/RestfulEntityBase.php).`

When you define a field with just a callback without a field property,
For example:
```php
$public_fields['phpVersion'] = array(
      'callback' => 'phpversion',
);
```
Then you need to use `empty()`.

Scenario: `$value['property'] = FALSE`.
  * Before: `if (!$value['property'])` returns `TRUE`;
  * After: `if (!isset($value['property']))` returns `FALSE`. :red_circle: 
  * After: `if (!empty($value['property']))` returns `TRUE`.