Fix transform embeddeds with no columns but with nested embeddeds (mongodb)

[hoshinokanade]

Issue type:

[ ] question
[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[x] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[x] latest
[ ] @next
[ ] 0.2.17 (or put your version here)

Steps to reproduce or a small repository showing the problem:

export class Post {
  @Column()
  title: string

  @Column(type => ExtraInfo)
  extraInfo: ExtraInfo
}
export class ExtraInfo {
  @Column(type => EditHistory)
  lastEdit: EditHistory
}
export class EditHistory {
  @Column()
  message: string
}
...
await repo.save(somePost);
const [post] = await repo.find();

// UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'lastEdit' of undefined
at \src\query-builder\transformer\DocumentToEntityTransformer.ts
at addEmbeddedValuesRecursively

// extraInfo is not created!

After hours of trial without looking at the source code, I got a workaround.

export class ExtraInfo {
  @Column(type => EditHistory)
  lastEdit: EditHistory

  @Column()
   justNothing: number = 1
}
// clear the collection, insert another post with that additional field
await repo.save(anotherPost);
const [post] = await repo.find();
// works perfectly

This workaround does not make any sense it should not be an intentional design so it might be a bug indeed...

In addEmbeddedValuesRecursively, I discovered that the field may not be created if embedded.columns is empty array.
Then, when the foreach clause is skipped and it tries the enter into next recursion of addEmbeddedValuesRecursively the first argument may be undefined,
which cause the entity in that recursion to be undefined,
eventually causing unexpected error as we still got some columns deep deep inside.

Here I suggest to fix it by an extra check, so that it won't break any existing behavior.

[rustamwin]

Suggested change

	* fixed transform embeddeds with no columns but with nested embeddeds (mongodb) ([#4131](https://github.com/typeorm/typeorm/pull/44131))
	* fixed transform embeddeds with no columns but with nested embeddeds (mongodb) ([#4131](https://github.com/typeorm/typeorm/pull/4131))

[rustamwin]

Looks good

[pleerock]

Thanks guys!

[hoshinokanade]

Thanks @rustamwin for review.