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
diff --git a/conformance/failure_list_php_c.txt b/conformance/failure_list_php_c.txt
index b040561..eff09ff 100644
--- a/conformance/failure_list_php_c.txt
+++ b/conformance/failure_list_php_c.txt
@@ -43,3 +43,4 @@
 Required.Proto3.ProtobufInput.FloatFieldNormalizeQuietNan.JsonOutput
 Required.Proto3.ProtobufInput.FloatFieldNormalizeSignalingNan.JsonOutput
 Required.Proto3.ProtobufInput.ValidDataRepeated.FLOAT.JsonOutput
+Required.Proto3.JsonInput.RejectTopLevelNull
diff --git a/conformance/failure_list_ruby.txt b/conformance/failure_list_ruby.txt
index 651cb55..a36f056 100644
--- a/conformance/failure_list_ruby.txt
+++ b/conformance/failure_list_ruby.txt
@@ -65,3 +65,4 @@
 Required.Proto3.JsonInput.IgnoreUnknownJsonObject.ProtobufOutput
 Required.Proto3.JsonInput.IgnoreUnknownJsonString.ProtobufOutput
 Required.Proto3.JsonInput.IgnoreUnknownJsonTrue.ProtobufOutput
+Required.Proto3.JsonInput.RejectTopLevelNull
diff --git a/conformance/failure_list_ruby_mac.txt b/conformance/failure_list_ruby_mac.txt
index 3114cba..0abc412 100644
--- a/conformance/failure_list_ruby_mac.txt
+++ b/conformance/failure_list_ruby_mac.txt
@@ -70,3 +70,4 @@
 Required.Proto3.JsonInput.IgnoreUnknownJsonString.ProtobufOutput
 Required.Proto3.JsonInput.IgnoreUnknownJsonTrue.ProtobufOutput
 Recommended.Proto3.JsonInput.FieldMaskInvalidCharacter
+Required.Proto3.JsonInput.RejectTopLevelNull