Fix nullability warnings in RCTExceptionsManager (#24467)
Fix nullability warnings in RCTExceptionsManager (#24467)

Summary:
There are a number of nullability warnings in `RCTExceptionsManager` when building React Native for iOS with Xcode 10.2. This resolves them without changing the existing behavior.

See the warnings here:

![image](https://user-images.githubusercontent.com/1773641/56201323-89551380-6038-11e9-9998-b6a8d3d28f36.png)

[iOS] [Fixed] - Fix nullability warnings in RCTExceptionsManager
Pull Request resolved: https://github.com/facebook/react-native/pull/24467

Differential Revision: D14973485

Pulled By: cpojer

fbshipit-source-id: 2fac9f067ac9418397c3913760a2403f9a2cc147

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