fix(orm): fix useJoin JoinDatabaseQuery generic for optional properties

[timvandam]

Summary of changes

Whenever doing a useJoin on some class property it uses T[K], but when T[K] is optional/can be undefined this leads to a return type being JoinDatabaseQuery<SomeEntity | undefined, this>, making it impossible to chain useJoins any further, as ReferenceFields<SomeEntity | undefined> resolves to undefined`.

This fixes that by using Exclude<T[K], undefined>

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[timvandam]

Closes #366

[marcj]

Fixed in a different way via https://github.com/deepkit/deepkit-framework/commit/e229380fd8fc14b4f4e38a25294a79a124534522