AFAIK usually sudo doesn't let you enter an empty string as a password, even when the actual password is empty. So that is what you might be experiencing.

root IS an admin in macOS. You just cannot use root the same way you expect root to work in other Unix OS' without changing stuff in the BIOS (or whatever it's called for Apple) but you can still do most admin things with root. However root does get admin privileges, but when it comes to some system directories he is banned till you "fix" root privileges.

Edit:

See: https://stackoverflow.com/a/33272796 for a bit more information of what I mean.