[ReactNative] Use RCTNullIfNill and (id)kCFNull

Summary:
@public

Use consistent `null` handling:

`value || null`                ->  `RCTNullIfNil(value)`
`value == null ? nil : value`  ->  `RCTNilIfNull(value)`
`[NSNull null]`                ->  `(id)kCFNull`

Test Plan: The tests should be enough.

Libraries/Network/RCTDataManager.m

@@ -436,7 +436,7 @@ RCT_EXPORT_MODULE()
 
     NSArray *responseJSON = @[
       request.requestID,
-      error.localizedDescription ?: [NSNull null],
+      RCTNullIfNil(error.localizedDescription),
     ];
 
     [_bridge.eventDispatcher sendDeviceEventWithName:@"didCompleteNetworkResponse"