Allow asking for any variant on an ab_test

If ab config changes variant configuration for an ab test,
ab lib used to throw errors if `ab_test.some_variant?` code
was invoked. Now it returns false.

Java/Objective-C implementations seem to behave in similar way.

cc @mmozuras 
Is there some `logger` instance I could use to send messages if non-existing variants are used?
On the other hand, I don't think there's any logging on mobile. And this would generate quite a lot of noise if we would keep the "update ab test to single variant to enable features" approach.

In general analytics dashboards should show exposure numbers/variant which should be enough to detect issues?