Add a new conformance test for top level JSON null (#5966)

* Add a new conformance test for top level JSON null
diff --git a/conformance/binary_json_conformance_suite.cc b/conformance/binary_json_conformance_suite.cc
index 8cb9bf7..0995d35 100644
--- a/conformance/binary_json_conformance_suite.cc
+++ b/conformance/binary_json_conformance_suite.cc
@@ -2452,6 +2452,8 @@
         "unknown": {"a": 1}
       })",
       "");
+
+  ExpectParseFailureForJson("RejectTopLevelNull", REQUIRED, "null");
 }
 
 }  // namespace protobuf