Merge pull request #27579 from fengyuentau:4x/core/filestorage

core: support parsing null in json parser in FileStorage #27579

Fixes https://github.com/opencv/opencv/issues/27578

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/core/test/test_io.cpp

@@ -1545,6 +1545,30 @@ TEST(Core_InputOutput, FileStorage_format_yml_gz)
     EXPECT_EQ(FileStorage::FORMAT_YAML, fs.getFormat());
 }
 
+TEST(Core_InputOutput, FileStorage_json_null_object)
+{
+    std::string test =
+        "{ "
+            "\"padding\": null,"
+            "\"truncation\": null,"
+            "\"version\": \"1.0\""
+        "}";
+    FileStorage fs(test, FileStorage::READ | FileStorage::MEMORY);
+
+    ASSERT_TRUE(fs["padding"].isNone());
+    ASSERT_TRUE(fs["truncation"].isNone());
+    ASSERT_TRUE(fs["version"].isString());
+
+    ASSERT_EQ(fs["padding"].name(), "padding");
+    ASSERT_EQ(fs["truncation"].name(), "truncation");
+    ASSERT_EQ(fs["version"].name(), "version");
+
+    ASSERT_EQ(fs["padding"].string(), "");
+    ASSERT_EQ(fs["truncation"].string(), "");
+    ASSERT_EQ(fs["version"].string(), "1.0");
+    fs.release();
+}
+
 TEST(Core_InputOutput, FileStorage_json_named_nodes)
 {
     std::string test =