Fabric: `ContextContainer::findInstance` returns an optional intead of throwing an exception
Fabric: `ContextContainer::findInstance` returns an optional intead of throwing an exception

Summary: Sometimes we don't know for sure if `ContextContainer` has a value or not (and that's perfectly legit use case). In those cases now we can use `findInstance` method that returns an optional intead of throwing an exeption.

Reviewed By: JoshuaGross

Differential Revision: D15039137

fbshipit-source-id: 95ba8cc7b76e37d1bd17e18c0098e56350ff3fef

pandiaraj44/react-native,hoangpham95/react-native,facebook/react-native,exponentjs/react-native,pandiaraj44/react-native,exponentjs/react-native,hoangpham95/react-native,exponent/react-native,arthuralee/react-native,javache/react-native,myntra/react-native,javache/react-native,exponent/react-native,myntra/react-native,exponentjs/react-native,pandiaraj44/react-native,janicduplessis/react-native,hammerandchisel/react-native,myntra/react-native,facebook/react-native,janicduplessis/react-native,pandiaraj44/react-native,myntra/react-native,exponentjs/react-native,exponent/react-native,javache/react-native,janicduplessis/react-native,exponentjs/react-native,myntra/react-native,pandiaraj44/react-native,hoangpham95/react-native,arthuralee/react-native,pandiaraj44/react-native,pandiaraj44/react-native,janicduplessis/react-native,exponent/react-native,hammerandchisel/react-native,hoangpham95/react-native,exponentjs/react-native,facebook/react-native,hammerandchisel/react-native,exponentjs/react-native,javache/react-native,arthuralee/react-native,myntra/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,hammerandchisel/react-native,hoangpham95/react-native,arthuralee/react-native,exponent/react-native,exponent/react-native,hammerandchisel/react-native,pandiaraj44/react-native,hoangpham95/react-native,javache/react-native,facebook/react-native,exponent/react-native,facebook/react-native,myntra/react-native,hammerandchisel/react-native,arthuralee/react-native,janicduplessis/react-native,janicduplessis/react-native,myntra/react-native,facebook/react-native,hoangpham95/react-native,hammerandchisel/react-native,facebook/react-native,myntra/react-native,hoangpham95/react-native,hammerandchisel/react-native,facebook/react-native,javache/react-native,exponentjs/react-native,facebook/react-native,exponent/react-native