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 property,
For example:
```php
$public_fields['phpVersion'] = array(
      'callback' => 'phpversion',
);
```