fix a crash when using autocomplete on enums

[akiraveliara]

this doesn't necessarily represent how this feature should look in the end, there's still some design holes and pitfalls (mainly, users not being forced to select an autocompleted value), but in the interim we should probably not crash

this is also documented on @OoLunar s WIP documentation as possible: https://github.com/DSharpPlus/DSharpPlus/blob/oolunar/docs/commands/docs/articles/commands/processors/slash_commands/choice_provider_vs_autocomplete.md?plain=1#L39-L42