(Hibernate) Can't add entity with many-to-many relationship - spring

Question

Basically, my goal is to have product and factory entity where relationship is many-to-many.
Factory must have some product, while product is not necessary associated with a factory. The problem is that when I try to add Product entity I get this error.
Caused by: org.hsqldb.HsqlException: integrity constraint violation: NOT NULL check constraint; SYS_CT_10103 table: PRODUCT column: FACT_ID
This is my Application class. I want to be able to save product to repository without giving it factory list but I got that error, so I tried adding what's in the comment but the same error remains.
#SpringBootApplication
public class Application implements CommandLineRunner {
#Autowired
private ProductRepo prodRepo;
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
#Override
#Transactional
public void run(String... strings) throws Exception {
Product one = new Product();
/*
List<Product> productList= new ArrayList<Product>();
productList.add(one);
SomeFactory factory_one = new SomeaFactory("one", productList);
List<Factory> factoryList = new ArrayList<Factory>();
factoryList.add(factory_one);
one.setFactoryList(factoryList);
*/
prodRepo.save(one);
}
Product.java
#Entity
#Table(name="PRODUCT")
public class Product {
private int id;
private List<Factory> factoryList = null;
public Product() {}
#Id
#Column(name = "PROD_ID")
#GeneratedValue(strategy = GenerationType.IDENTITY)
public int getId() { return id; }
public void setId(int id) { this.id = id; }
#ManyToMany(fetch = FetchType.LAZY, mappedBy = "productList", targetEntity = Factory.class)
public List<Factory> getFactoryList() {
return factoryList;
}
public void setFactoryList(List<Factory> f) {
this.factoryList = f;
}
}
SomeFactory.java
#Entity
#Table(name ="SOME_FACT")
public class SomeFactory extends Factory {
private String name;
private List<Product> productList;
public SomeFactory() {}
public SomeFactory(String name, List<Product> products) {
this.name = name;
this.productList = products;
}
#Column(name = "FACT_NAME", nullable = false)
public String getName() { return name; }
public void setName(String name) { this.name = name; }
#ManyToMany
#JoinTable(name = "PRODUCT",
joinColumns = #JoinColumn(name = "FACT_ID", nullable = true),
inverseJoinColumns = #JoinColumn(name = "PROD_ID", nullable = false))
public List<Product> getProductList() {
return productList;
}
public void setProductList(List<Product> products) {
this.productList = products;
}
}
Factory.java
#Entity
#Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Factory {
int id;
public Factory() {}
#Id
#Column(name="FACT_ID")
#GeneratedValue(strategy = GenerationType.TABLE)
int getId() { return id; }
void setId(int id) { this.id = id; }
}