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',
);
```
`empty()` is not needed because the entity validation is executed here to validate the entity field data.
The entity validate should throw you an error if the field is required but empty.