feat: improve handling of empty/missing relations during insert/update

[michalsn]

Description
This PR improves how the system handles related data during insert and update operations.

Previously, if a relation was missing or empty, it could lead to unexpected behavior or errors.

With this change, the insert/update process now gracefully skips missing relations, ensuring that operations continue smoothly without breaking existing logic.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide