Does Staraptor's "Cyclone Slash" attack do any damage if the opponent has no Benched Pokemon to switch with their Active Pokemon?
Yes, it does the damage even if no Pokemon are switched.
Source: BREAKthrough FAQ (2015-11-19), TPCi Rules Team (2015-11-19)

Relevant cards

Note: All applicable cards may not be listed below.
XY8_127