If you have access to root, login to root and reinstall sudo from pacman, if not then you'll probably have to chroot into it and install it from there.

No it is not ment to be empty. You will need to reinstall it to fix that.

It's not meant to be empty. You could fix it by mounting your root partition in a live environment and copying over the sudo binary from the live disk, but without knowing why this happened I'd personally be a bit nervous that other binaries are corrupted as well, so I'd maybe consider reinstalling the OS (preserving the home partition/folder ofc). You could also probably get away with just reinstalling coreutils and fixing any other issues as they crop up if and when they do.