You cannot activate an iPhone without a sim. The sim does not have to be
associated with an active cellular account; however, if the iPhone is locked to a
particular wireless provider, the sim must be from that wireless provider.
Ask an wireless provider for an inactive sim (if iPhone is locked to a
wireless provider, ask that wireless provider) or borrow an appropriate
sim from a friend/relative/co-worker for the few minutes it will take to
activate the iPhone.
You will need a sim if you restore the iPhone or update the iOS version.