Adding nonnull macro to component event (#24523)

Summary:
We don't accept any nil parameters, so let compiler check it~

cc. janicduplessis cpojer

[iOS] [Fixed] - Adding nonnull macro to component event
Pull Request resolved: https://github.com/facebook/react-native/pull/24523

Differential Revision: D15022399

Pulled By: shergin

fbshipit-source-id: 8165d49c62d69404aac7ea0ba4291e397d962617

React/Base/RCTComponentEvent.h

@@ -7,6 +7,8 @@
 
 #import <React/RCTEventDispatcher.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 /**
  * Generic untyped event for Components. Used internally by RCTDirectEventBlock and
  * RCTBubblingEventBlock, for other use cases prefer using a class that implements
@@ -16,4 +18,6 @@
 
 - (instancetype)initWithName:(NSString *)name viewTag:(NSNumber *)viewTag body:(NSDictionary *)body;
 
+NS_ASSUME_NONNULL_END
+
 @end