fix: make dropdown work with a singular choice

[TymekDev]

Issue #60

Description

Prior to this change a singular value wasn't serialized as an array. This lead to JS errors when trying to set options.

Definition of Done

• [x] The change is thoroughly documented.
• [ ] The CI passes (R CMD check, linter, unit tests, spelling).
• [x] Any generated files have been updated (e.g. .Rd files with roxygen2::roxygenise())