update nullability of an existing dictionary

* Now that the OneSignalClientError type contains a nullable responseHeaders property, there is no need to default a null dictionary to be an empty dictionary
* This default was recently added to prevent a crash that is no longer an issue after these changes.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalCore/Source/API/OneSignalClient.m

@@ -197,7 +197,7 @@ - (void)handleJSONNSURLResponse:(NSURLResponse*)response data:(NSData*)data erro
 
     NSHTTPURLResponse* HTTPResponse = (NSHTTPURLResponse*)response;
     NSInteger statusCode = [HTTPResponse statusCode];
-    NSDictionary *headers = [HTTPResponse allHeaderFields] ?: @{};
+    NSDictionary *headers = [HTTPResponse allHeaderFields]; // can be null
     NSError* jsonError = nil;
     NSMutableDictionary* innerJson;