[Form] Remove "value" attribute on empty_value option

Today we faced a very strange issue with the newest Blackberry 10 browser, it was not submitting our forms. Finally we found that in a ```select``` element, if you have a disabled option, it can't have a value or the HTML5 validator will crash and won't submit the form. Of course, setting the ```novalidate``` option for the whole form also solved the issue.

Although I know this must be an issue with the WebKit version the BB10 has, it can easily be solved in symfony with this change. In fact, it does make sense since we already have a disabled option with no value if the ```preferred_choices``` are not empty and a ```separator``` is set