title: typeorm error: null value in column violates not-null: allow null values ​in a fk
body: <p>Hello i have two entities employee and department where a department has 0 or 1 manager
  I try to add a department without a manager and I am getting this error:</p>
  <blockquote>
  <p>error: error: null value in column &quot;manager_id&quot; violates not-null
  constraint</p>
  </blockquote>
  <p>i try add one departament with only name but without manager
  how could I allow null values ​​in a fk</p>
  <p>entity Employee:</p>
  <pre><code>@Entity({ name: 'employees' })
  export class Employee extends SharedProp {
    constructor(firstName: string, lastName: string) {
      super();
      this.firstName = firstName;
      this.lastName = lastName;
    }
    @PrimaryGeneratedColumn('uuid')
    id: string;
  
    @Column({ name: 'matricula', nullable: false })
    matricula: string;
  
    @Column({ name: 'first_name', nullable: false })
    firstName: string;
  
    @Column({ name: 'last_name', nullable: false })
    lastName: string;
  
    @Column()
    user_id: string;
  
    @Column()
    departament_id: string;
  
    @OneToOne(() =&gt; Departament, departament =&gt; departament.manager)
    @ManyToOne(
      () =&gt; Departament,
      (departament: Departament) =&gt; departament.employees,
    )
    @JoinColumn({ name: 'departament_id' })
    departament: Departament;
  
    @OneToOne(() =&gt; User)
    @JoinColumn({ name: 'user_id' })
    user: User;
  }
  </code></pre>
  <p>entity departament:</p>
  <pre><code>@Entity({ name: 'departaments' })
  export class Departament {
    @PrimaryGeneratedColumn('uuid')
    id: string;
  
    @Column()
    departament_name: string;
  
    @Column()
    manager_id: string;
  
    @OneToOne(() =&gt; Employee, employee =&gt; employee.departament)
    @JoinColumn({ name: 'manager_id' })
    manager: Employee;
  
    @OneToMany(() =&gt; Employee, (employee: Employee) =&gt; employee.departament)
    employees: Array&lt;Employee&gt;;
  
    @CreateDateColumn({ name: 'created_at' })
    createdAt: Date;
  
    @UpdateDateColumn({ name: 'updated_at' })
    UpdatedAt: Date;
  }
  </code></pre>