The following is no longer a syntax error:
```
case foo # expands to `foo`
end

case foo # expands to `foo; bar`
else
  bar
end
```

While an empty case doesn't make much sense in regular code, it is
useful for automatically generated `when` branches (for example in a macro)
and avoids extra effort to prevent empty case nodes.

This also fixes a parsing error for `case` without condition (`case;
when true; end`).