[release/10.0-preview6] Fix HttpClientHandler.CreateNativeHandler implementation (#116767)

Co-authored-by: Jeremy Koritzinsky <jekoritz@microsoft.com>
Co-authored-by: Jeremy Koritzinsky <jkoritzinsky@gmail.com>
Fix #116698

Author: github-actions[bot]

src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.InvokeNativeHandler.cs

@@ -332,11 +332,10 @@ private void SetCredentials(ICredentials? value)
 
         private static HttpMessageHandler CreateNativeHandler()
         {
-            return (HttpMessageHandler)CallNative();
+            return CallNative(null);
 
             [UnsafeAccessor(UnsafeAccessorKind.StaticMethod, Name = "GetHttpMessageHandler")]
-            [return: UnsafeAccessorType(NativeHandlerType)]
-            static extern object CallNative();
+            static extern HttpMessageHandler CallNative([UnsafeAccessorType(GetHttpMessageHandlerType)] object? _);
         }
     }
 }