Fix: trigger GenericObject dropdown fields migration

[Rom1-B]

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 !43878
• The migration guard that detects legacy GenericObject dropdown fields read the wrong result key from the count query, so it always evaluated to false.
• As a result, GenericObject dropdown fields were never migrated to CustomAsset dropdown fields, and affected fields displayed the wrong label on item forms.

Screenshots (if appropriate):

[MyvTsv]

That looks good to me. I'll wait for the client's feedback before approving the fix.