[Config] Some NodeDefinition::cannotBeEmpty() should throw an exception

Several  `NodeDefinition::cannotBeEmpty()` has been deprecated in #15170 in favour of [throwing an exception](https://github.com/symfony/symfony/pull/15170#issuecomment-119538615) in 3.0.
These deprecations have been removed in #15685 but the exceptions haven't been added.

So my question is: Is it too late to add them in 3.0 ?
