Fix(Order): handle multigroup assign

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• Fixes : Order plugin – “Generate associated material” shows empty page after reception #543

Prevent Twig errors during asset generation caused by group fields that are now multi-select for assets using the AssignableItem trait.

Screenshots (if appropriate):

[Rom1-B]

Please update changelog