Allow `switch` with zero arguments

I think one should be able to `switch` on a variable that may or may not be set:

```fish
switch $LANG
    case 'en_US*'
...
end
```

If `$LANG` is unset:

`fish: switch: Expected exactly one argument, got 0`

(Same with command substitutions which may sometimes print nothing.)

I think it should just silently short circuit and do nothing.