Allow fallthrough on empty cases

[Sainan]

It's a bit strange, but there's no logical reason for why it should be a parse error. Fixes #859.

[XmiliaH]

How about:

switch 1 do
    case 1:
        --[[@fallthrough]]
    default:
end

and I think this allows

switch 1 do
        --[[@fallthrough]]
    case 1:
end