fix(persistence): keep many-to-many junction rows when the relation is not loaded - #12711

Fixes #12710

Description of change

Saving several entities in one save() call (or via a cascade) silently deleted every many-to-many junction row of any entity in the batch whose relation property was not loaded, whenever another entity in the same persistence group had that relation loaded.

Root cause: the prefer-nullish-coalescing style refactor (cab8fcb, #12340) changed ManyToManySubjectBuilder.buildForSubjectRelation from

if (relatedEntities === null) relatedEntities = []
if (!Array.isArray(relatedEntities)) return

to

relatedEntities ??= []
if (!Array.isArray(relatedEntities)) return

??= also matches undefined, so an unloaded relation — which previously fell through to the !Array.isArray guard and was skipped — became "relation set to empty, remove all junction rows". The removal only materializes when another subject in the same group has the relation loaded (that makes SubjectDatabaseEntityLoader fetch the database-side relation ids for the whole target group), which is why single-entity saves look fine and the data loss surfaces on batch saves and cascades. The same refactor kept the explicit undefined → return guard in the sibling OneToManySubjectBuilder — only the many-to-many builder lost it.

Fix: restore the undefined → skip guard, mirroring OneToManySubjectBuilder. null still means "remove all bindings" and non-array values (e.g. lazy-relation promises) are still skipped, so the documented 1.0 semantics are all preserved:

• undefined (not loaded) → junction rows left intact (this was broken)
• null / [] → all junction rows removed (unchanged, still covered by the existing tests in the same file)

Verification:

• New regression test in test/functional/persistence/many-to-many/persistence-many-to-many.test.ts: two posts saved in one call, one loaded with its categories and one without; asserts the unloaded one keeps its junction row. It fails on master (the junction row is deleted) and passes with this fix.
• Full local sqlite test leg passes (3039 passing).
• A standalone reproduction script is included in Batch save() silently deletes many-to-many junction rows of entities saved without their relation loaded (v1.0 regression) #12710; against this branch it shows both junction rows surviving.

Pull-Request Checklist

• Code is up-to-date with the master branch
• This pull request links a relevant issue using a closing keyword:
  Fixes #NNNN, Closes #NNNN, or Resolves #NNNN — Fixes Batch save() silently deletes many-to-many junction rows of entities saved without their relation loaded (v1.0 regression) #12710
• There are new or updated tests validating the change (tests/**.test.ts)
• Documentation has been updated to reflect this change (docs/docs/**.md) — N/A (restores the documented behavior from the 1.0 upgrade guide; no documented behavior changes)