* Introduce a new Enumerator for the SafetyIntegrityLevel annotation
  that allows to specify unset values by pointing to null. In case a
  value is set for an IModelElement, the attribute points to the
  standard specific enum.
* Update the haveDirectChildsSpecifiedSIL(...) method to consider only
  directly set safety levels of IModelElements.

Issue-Ref: 3859
Issue-Url: https://af3-developer.fortiss.org/issues/3859

Signed-off-by: Alexander Diewald <diewald@fortiss.org>