Don't freeze null

Reviewed By: acdlite

Differential Revision: D7567032

fbshipit-source-id: ea9c826d572371748c6e9b5defbb92b427d83bee

Libraries/StyleSheet/StyleSheet.js

@@ -366,9 +366,11 @@ module.exports = {
     if (__DEV__) {
       for (const key in obj) {
         StyleSheetValidation.validateStyle(key, obj);
+        if (obj[key]) {
           Object.freeze(obj[key]);
         }
       }
+    }
     return obj;
   },
 };