A typo in the fish completions (that was likely copied to the zsh and
bash completions) prevented CLI actions from using enums as option
values because the completions tried to access non-existent fields from
type introspection. This doesn't cause any problems _now_ because no CLI
action uses an enum as an option value. However as soon as you try and
add one the completions fail to compile.
This patch fixes the incorrect field reference. It also adds the ability
to have _optional_ enums as option values.
