Eintrag(assertionCreator: ((Assert<T>) -> Unit)?)
Parameter object to express a Group with a single identification lambda.
In case null is used for the identification lambda then it is expected that the corresponding entry
is null as well.
assertionCreator - The identification lambda identifying the entry where an entry is considered
to be identified if it holds all Assertions the lambda might create or if it is null in case
assertionCreator is defined as null.