change add_rule raise error to nil when subject nil

[ayumu838]

This PR change raise error to nil when subject is nil

Because my development service down when not exist subject active model.
I expect to nothing happen in this case.

---

now

action	subject	active model	result
manage	Foo	None	raise error

expected

action	subject	active model	result
manage	Foo	None	nothing

[coorasse]

We don't support defining abilities without a subject anymore. See #489