[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/WebSocket/RCTWebSocketManager.m

@@ -13,6 +13,7 @@
 #import "RCTEventDispatcher.h"
 #import "RCTSRWebSocket.h"
 #import "RCTSparseArray.h"
+#import "RCTUtils.h"
 
 @implementation RCTSRWebSocket (React)
 
@@ -107,7 +108,7 @@ RCT_EXPORT_METHOD(close:(NSNumber *)socketID)
 {
   [_bridge.eventDispatcher sendDeviceEventWithName:@"websocketClosed" body:@{
     @"code": @(code),
-    @"reason": reason ? reason : [NSNull null],
+    @"reason": RCTNullIfNil(reason),
     @"clean": @(wasClean),
     @"id": webSocket.reactTag
   }];