ManyToMany and Custom Typed PK

[mastercoding]

Describe the bug
When using a custom typed PK (see #1798), ManyToMany using a pivotTable doesn't work. The map returned from

await this.driver.loadFromPivotTable()

does not contain converted keys and thus the lookup fails.

map[entity.__helper.getSerializedPrimaryKey()] 

is undefined.

Stacktrace

TypeError: Cannot read property 'map' of undefined
    at EntityLoader.findChildrenFromPivotTable (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:232:69)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async EntityLoader.populateField (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:185:25)
    at async EntityLoader.populate (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:37:13)
    at async EntityLoader.populateField (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:203:9)
    at async EntityLoader.populate (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:37:13)
    at async SqlEntityManager.find (/app/node_modules/@mikro-orm/core/EntityManager.js:105:9)
    at async EntityLoader.populateMany (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:127:22)
    at async EntityLoader.populateField (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:187:9)
    at async EntityLoader.populate (/app/node_modules/@mikro-orm/core/entity/EntityLoader.js:37:13)

To Reproduce
Steps to reproduce the behavior:

Create the entities with the custom type from #1798.

export class Product {
  @PrimaryKey({ type: UuidBinaryType, name: 'uuid' })
  id: string;

 @ManyToMany({
    owner: true,
    entity: () => Field,
    pivotTable: 'product_order_field',
    joinColumn: 'product_id',
    inverseJoinColumn: 'order_field_id',
  })
  fields: Collection<Field> = new Collection<Field>(this);
}

export class Field {
  @PrimaryKey({ type: UuidBinaryType, name: 'uuid' })
  id: string;

  @Property()
  title: string;
}

2. Fetch products with its fields from the database.

Versions

Dependency	Version
node	14.16.1
typescript	4.2.3
@mikro-orm/core	4.5.6
@mikro-orm/mysql	4.5.6
@mikro-orm/nestjs	4.2.0