Fabian Homborg 6500765256 Allow switch with something that expands to nothing
Meaning empty variables, command substitutions that don't print
anything.

A switch without an argument

```fish
switch
   case ...
end
```

is still a syntax error, and more than one argument is still a runtime
error.

The none-argument matches either an empty-string `case ''` or a
catch-all `case '*'`.

Fixes #5677.

Fixes #4943.
2019-07-31 14:08:28 +02:00
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