Fixed GCDWebServerBodyReaderCompletionBlock not allowing null data

Fixed #434

GCDWebServer/Core/GCDWebServerResponse.h

@@ -33,7 +33,7 @@ NS_ASSUME_NONNULL_BEGIN
  *  The GCDWebServerBodyReaderCompletionBlock is passed by GCDWebServer to the
  *  GCDWebServerBodyReader object when reading data from it asynchronously.
  */
-typedef void (^GCDWebServerBodyReaderCompletionBlock)(NSData* data, NSError* _Nullable error);
+typedef void (^GCDWebServerBodyReaderCompletionBlock)(NSData* _Nullable data, NSError* _Nullable error);
 
 /**
  *  This protocol is used by the GCDWebServerConnection to communicate with