Fix RCTNullIfNil macro
Fix RCTNullIfNil macro

Summary:
`RCTNullIfNil()` can return nil in certain scenarios.

Example, given:
`#define RCTNullIfNil(value) (value ?: (id)kCFNull)`
`RCTNullIfNil(nil == nil ? nil : @"lol")`
expanded out
`nil == nil ? nil : @"lol" ?: (id)kCFNull`

`?:` takes precedence, so reduced:
`nil == nil ? nil : @"lol"`
`nil`

Changelog: [iOS] [Fixed] Fixed longstanding bug where RCTNullIfNil() can return nil

Reviewed By: RSNara

Differential Revision: D17943530

fbshipit-source-id: 8c6e3dd2d86cbc8ff1fcbef732674835a312ef26

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