Fix Image component crashing when uri is null (#28061)

Summary:
This fixes #28060 with a fix similar to f940e7c

## Changelog

[iOS] [Fixed] - Fix Image component crashing when uri is null
Pull Request resolved: #28061

Test Plan:
Run an app on iOS that renders `<Image source={{ uri: null }} />`. It should not crash.
![error](https://user-images.githubusercontent.com/4928274/74492398-6bb23e00-4ed7-11ea-8482-664e1786bba8.png)

Reviewed By: sammy-SC

Differential Revision: D20080680

Pulled By: PeteTheHeat

fbshipit-source-id: 119766a4d7eb6804ffd668418a1f158a712a4fe0

React/Base/RCTConvert.m

@@ -75,7 +75,7 @@ + (NSIndexSet *)NSIndexSet:(id)json
 
 + (NSURL *)NSURL:(id)json
 {
-  NSString *path = [self NSString:json];
+  NSString *path = [self NSString:RCTNilIfNull(json)];
   if (!path) {
     return nil;
   }