Summary: public

In iOS < 9, inserting a nil object into NSMutableDictionary crashes. It is valid for come components to return a nil shadowView (e.g. ART nodes), and this was crashing on iOS 8.

Reviewed By: jspahrsummers

Differential Revision: D2658309

fb-gh-sync-id: 7abf9273708cc03c3b6307b69ba11c016b471fbe