fix: return 'null' (instead of 'undefined') on lazy relations that have no results

[securedirective]

Description of change

Fix an issue where a lazy relation returns a Promise that resolves to 'undefined'. As implied by
documentation, if the database has a NULL or no results, the result should be a literal 'null'
in Javascript. Also added units tests to hilight the three scenarios in which this occurred.

This PR is to fix reported issue #7146.

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

[securedirective]

Prior to applying the fix, here's what my unit tests showed:

  github issues > #7146 Lazy relations resolve to 'undefined' instead of 'null'
    lazy-loaded relations
      1) should return null if ManyToOne relation has NULL in database
      2) should return null if OneToOne+JoinColumn relation has NULL in database
      3) should return null if OneToOne relation has NULL in database
    lazy-loaded relations included in 'relations' find option
      ✓ should return null if ManyToOne relation has NULL in database
      ✓ should return null if OneToOne+JoinColumn relation has NULL in database
      ✓ should return null if OneToOne relation has NULL in database
    eager-loaded relations
      ✓ should return null if ManyToOne relation has NULL in database
      ✓ should return null if OneToOne+JoinColumn relation has NULL in database
      ✓ should return null if OneToOne relation has NULL in database


  6 passing (5s)
  3 failing

  1) github issues > #7146 Lazy relations resolve to 'undefined' instead of 'null'
       lazy-loaded relations
         should return null if ManyToOne relation has NULL in database:
     AssertionError: expected undefined to be null
      at /home/kenneth/typeorm/test/github-issues/7146/issue-7146.ts:31:51
      at step (node_modules/tslib/tslib.js:141:27)
      at Object.next (node_modules/tslib/tslib.js:122:57)
      at fulfilled (node_modules/tslib/tslib.js:112:62)
      at processTicksAndRejections (node:internal/process/task_queues:93:5)

  2) github issues > #7146 Lazy relations resolve to 'undefined' instead of 'null'
       lazy-loaded relations
         should return null if OneToOne+JoinColumn relation has NULL in database:
     AssertionError: expected undefined to be null
      at /home/kenneth/typeorm/test/github-issues/7146/issue-7146.ts:37:55
      at step (node_modules/tslib/tslib.js:141:27)
      at Object.next (node_modules/tslib/tslib.js:122:57)
      at fulfilled (node_modules/tslib/tslib.js:112:62)
      at processTicksAndRejections (node:internal/process/task_queues:93:5)

  3) github issues > #7146 Lazy relations resolve to 'undefined' instead of 'null'
       lazy-loaded relations
         should return null if OneToOne relation has NULL in database:
     AssertionError: expected undefined to be null
      at /home/kenneth/typeorm/test/github-issues/7146/issue-7146.ts:43:50
      at step (node_modules/tslib/tslib.js:141:27)
      at Object.next (node_modules/tslib/tslib.js:122:57)
      at fulfilled (node_modules/tslib/tslib.js:112:62)
      at processTicksAndRejections (node:internal/process/task_queues:93:5)

After applying the fix:

  github issues > #7146 Lazy relations resolve to 'undefined' instead of 'null'
    lazy-loaded relations
      ✓ should return null if ManyToOne relation has NULL in database (71ms)
      ✓ should return null if OneToOne+JoinColumn relation has NULL in database
      ✓ should return null if OneToOne relation has NULL in database
    lazy-loaded relations included in 'relations' find option
      ✓ should return null if ManyToOne relation has NULL in database
      ✓ should return null if OneToOne+JoinColumn relation has NULL in database
      ✓ should return null if OneToOne relation has NULL in database
    eager-loaded relations
      ✓ should return null if ManyToOne relation has NULL in database
      ✓ should return null if OneToOne+JoinColumn relation has NULL in database
      ✓ should return null if OneToOne relation has NULL in database


  9 passing (6s)

[pleerock]

Seems reasonable. Thank you for contribution!

[securedirective]

I think that was the first time I've created a Pull Request on an open-source project, complete with unit tests, and had it accepted. Thank you for the experience!

[pekunicki]

This issue still occurs when you retrieve a newly created item via .save(...) 😞

[Isli-GD]

Did this PR introduce the bug #8582?
I am not sure about internals, so this is just a guess.
@securedirective can you elaborate on that?