fix: error where eager relations in a lazy-loaded entity would be undefined

[Bartmr]

Description of change

Given the scenario:

Entity A {
  id
  b: any type of relationship to Entity B | lazy
}

Entity B {
  id
  c: any type of relationship to something else | eager
  d: any type of relationship to something else | eager
}

this change intends to fix a bug that happens when lazy-loading a relationship with await entityA.b. All eager relations from Entity B are not loaded, and are set as undefined. This happens with all find methods and the Query Builder, breaking type-safety, specially for people using strictNullChecks in tsconfig.

Fixes #8328 #7841 (comment)

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

Thank you for contribution! 🎉