[Lainow]

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

• It fixes !47292
• Here is a brief description of what this PR does

Submitting a form with an empty multiple dropdown field question failed with a TypeError. An empty selection sends items_id = '', and formatRawAnswer() passed that value to getById() without casting it to an integer. The ids are now cast to integers and empty values are filtered out.

Screenshots (if appropriate):