[2.4.0] Checkmk Werk 16818 created: Formspec migration for specific CascadingSingleChoices values

[//]: # (werk v2) # Formspec migration for specific CascadingSingleChoices values key | value ---------- | --- date | 2024-05-24T07:19:53+00:00 version | 2.4.0b1 class | fix edition | cre component | checks level | 1 compatible | yes This only affects plugin developers using the new rulesets API. When migrating a `None` value to a tuple suitable for a `CascadingSingleChoice` formspec, the migration function was not applied.