Embeddables - undefined instead of null, smart query filter by null not allowed

[ml1nk]

Describe the bug
If an embeddable is nullable and all columns of it are null, then the property is expected to be null but is currently undefined (forceUndefined is not set). In addition I expected { a: null } (see test case) to work, but got an exception.

Stack trace
await orm.em.findOne(B, { a: null })

TypeError: Cannot convert undefined or null to object
        at Function.keys (<anonymous>)

      65 |     }
      66 |
    > 67 |     const operator = Object.keys(payload[item]).some(f => Utils.isOperator(f));
         |                             ^
      68 |
      69 |     if (operator) {
      70 |       throw ValidationError.cannotUseOperatorsInsideEmbeddables(entityName, prop.name, payload);

      at Function.createObjectItemNode (packages/knex/src/query/CriteriaNodeFactory.ts:67:29)
      at packages/knex/src/query/CriteriaNodeFactory.ts:52:22
          at Array.reduce (<anonymous>)
      at Function.createObjectNode (packages/knex/src/query/CriteriaNodeFactory.ts:51:41)
      at Function.createNode (packages/knex/src/query/CriteriaNodeFactory.ts:22:19)
      at QueryBuilder.where (packages/knex/src/query/QueryBuilder.ts:278:46)
      at SqliteDriver.find (packages/knex/src/AbstractSqlDriver.ts:94:8)
      at SqliteDriver.findOne (packages/knex/src/AbstractSqlDriver.ts:136:28)
      at SqlEntityManager.findOne (packages/core/src/EntityManager.ts:535:34)
      at SqlEntityManager.findOneOrFail (packages/core/src/EntityManager.ts:580:16)
      at Object.<anonymous> (tests/issues/GHXXXX2.test.ts:54:15)

To Reproduce

import { Embeddable, Embedded, Entity, PrimaryKey, Property } from '@mikro-orm/core';
import { MikroORM } from '@mikro-orm/sqlite';

@Embeddable()
export class A {

  @Property()
  test!: number;

}

@Entity()
export class B {

  @PrimaryKey()
  id!: number;

  @Embedded({ entity: () => A, object: false, nullable: true })
  a!: A  | null;

}

describe.only('GH issue XXXX', () => {

  let orm: MikroORM;

  beforeAll(async () => {
    orm = await MikroORM.init({
      entities: [A, B],
      dbName: ':memory:'
    });
    await orm.getSchemaGenerator().ensureDatabase();
    await orm.getSchemaGenerator().dropSchema();
    await orm.getSchemaGenerator().createSchema();

    orm.em.create(B, { a: null });
    await orm.em.flush();
    orm.em.clear();
  });

  afterAll(() => orm.close(true));

  test('embedded should be null', async () => {
    const f = await orm.em.getRepository(B).findAll()
    expect(f.length).toBe(1)
    expect(f[0].a).toBeNull(); // undefined
  });

  test('get embedded by null', async () => {

    const a = await orm.em.findOne(B, { a: { test: null } })
    expect(a).not.toBeNull()

    const f = await orm.em.findOne(B, { a: null }) // throws 
    expect(f).not.toBeNull()

  });
});

Versions

Dependency	Version
mikro-orm	cbc0c50