BUG: User-defined fields of type `select` with `allow_multiple=true` are not getting indexed in Typesense

[ajolipa]

At least in some cases, it appears that select UDFs that allow multiple values are returning nil from the convert_value method in the user-defined-fields gem, because the value passed is already type Array and thus JSON.parse is giving an error. https://github.com/performant-software/user-defined-fields/blob/9799f151ab51506eef42177c756b762060a9838a/app/services/user_defined_fields/converter.rb#L24