@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Parent
{
    @Id
    int id;

    @Column(nullable = true)
    String string;

    public int getId() { return this.id; }
    public String getString() { return this.string; }
}

@Entity
public class CanBeNull
    extends Parent
{
}

@Entity
public class CannotBeNull
    extends Parent
{
    // omitted for demo: @Column(nullable = false)
    @NotNull
    @Override
    public String getString() { return this.string; }
}

CREATE TABLE cannot_be_nulls
(
  id integer NOT NULL,
  string character varying(255) NOT NULL,
  CONSTRAINT cannot_be_nulls_pkey PRIMARY KEY (id)
)
CREATE TABLE can_be_nulls
(
  id integer NOT NULL,
  string character varying(255) *************NOT NULL*******************,
  CONSTRAINT can_be_nulls_pkey PRIMARY KEY (id)
)