Form\Element\Select multiple is always required

The Select Element is always required. The Validation fails (required) if the multiple select is unselected. It is not possible to send an empty list.

I added the option for an hidden element. If nothing is selected the empty hidden field is send (required). 

Select Element has the new Option 'use_hidden_element' and 'unselected_value'.
The FormSelect Helper generates an hidden element before the select element.

Reference to other Issues.
#4750, #5842, https://github.com/doctrine/DoctrineModule/issues/215