Unify unspecified arguments None vs ()

[JorisVincent]

Observed behavior

Some functions can leave out arguments by specifying (explicit or implicit) None. For others, e.g. rings_generalized(target_indices=None) raises errors -- instead it expects ()` (or some other empty sequence).
This is confusing.

Expected behavior

In all cases where an argument is optional, it should accept None. In cases where () would work, both should work.