[2.0][Form] DateTimeToTimestampTransformer should return empty string

Bug fix: yes
Feature addition: no
Backwards compatibility break: no*
Symfony2 tests pass: yes

According to note in [`DataTransformerInterface`](http://api.symfony.com/master/Symfony/Component/Form/DataTransformerInterface.html#method_transform) :

> By convention, transform() should return an empty string if NULL is passed.