Eagerly initialize to-n ArrayLists

[Clashsoft]

We can do away with the to-n association and attribute backing lists being nullable, and declare it initialized from the start:

private final java.util.ArrayList<Student> students = new java.util.ArrayList<>();

ArrayList creation is cheap, it does not even instantiate a backing array until the first element is added.

[azuendorf]

Please measure the memory overhead that would be introduced by this change.