Require at least one value in diff-root and meta-diff arguments

Require at least one value in diff-root and meta-diff arguments. None of the changed ones make sense to be passed without any value.

Not sure how to extensively test this so I'll just add full-unit-tests to make me feel better 😊 It hopefully won't affect ART tests at all.

/cc @jchapman @nstyles @jcatmore @tsulaia @amete @tcuhadar @simmons