Fix sync http nullable parameter problem.

PiperOrigin-RevId: 335849279
diff --git a/lib/src/request/sync_http_request_client.dart b/lib/src/request/sync_http_request_client.dart
index 104c6e4..0e3f615 100644
--- a/lib/src/request/sync_http_request_client.dart
+++ b/lib/src/request/sync_http_request_client.dart
@@ -23,7 +23,7 @@
         httpRequest = SyncHttpClient.postUrl(resolve(request.uri!));
         httpRequest.headers.contentType =
             ContentType('application', 'json', charset: 'utf-8');
-        httpRequest.write(request.body!);
+        httpRequest.write(request.body);
         break;
       case HttpMethod.httpDelete:
         httpRequest = SyncHttpClient.deleteUrl(resolve(request.uri!));