Add @Nullable support to set required=false on property constraints

[jeroenvs]

It should be possible to add the @nullable annotation on fields, making them required=false. Overriding the database constraint.

    @Nullable
    @ElementCollection
    @CollectionTable(name = "cars_inspections", joinColumns = @JoinColumn(name = "cars_id"))
    private List<Inspection> inspections = new ArrayList<>();