Annotate valuePublishedByPlugin with nullable (flutter#19267)

Author: ened

shell/platform/darwin/ios/framework/Headers/FlutterPlugin.h

@@ -415,7 +415,7 @@ typedef enum {
  *   nothing has been published. Will be `nil` if the plugin has not been
  *   registered.
  */
-- (NSObject*)valuePublishedByPlugin:(NSString*)pluginKey;
+- (nullable NSObject*)valuePublishedByPlugin:(NSString*)pluginKey;
 @end
 
 #pragma mark -