fix(dropdown): handle empty default value to prevent error

[MyvTsv]

• 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 !41833
• When creating a multiple-choice field without a default value, the default value is correctly stored in the database as an empty array.

However, when adding a new item, the field value is transmitted as an empty string, resulting in an inconsistent value being stored for the created object.

This inconsistency then causes an error when displaying the form, as the stored value does not match the expected type for a multiple-choice field.

This pull request corrects this behavior to ensure the consistency of the stored data type and avoid the form display error.