Add tests to demonstrate json parsing for null Timestamp and Duration types

conformance/conformance_test.cc

@@ -1771,6 +1771,10 @@ bool ConformanceTestSuite::RunSuite(ConformanceTestRunner* runner,
       R"({"repeatedDuration": ["1.5s", "-1.5s"]})",
       "repeated_duration: {seconds: 1 nanos: 500000000}"
       "repeated_duration: {seconds: -1 nanos: -500000000}");
+  RunValidJsonTest(
+      "DurationNull", REQUIRED,
+      R"({"optionalDuration": null})",
+      "");
 
   ExpectParseFailureForJson(
       "DurationMissingS", REQUIRED,
@@ -1840,6 +1844,10 @@ bool ConformanceTestSuite::RunSuite(ConformanceTestRunner* runner,
       "TimestampWithNegativeOffset", REQUIRED,
       R"({"optionalTimestamp": "1969-12-31T16:00:00-08:00"})",
       "optional_timestamp: {seconds: 0}");
+  RunValidJsonTest(
+      "TimestampNull", REQUIRED,
+      R"({"optionalTimestamp": null})",
+      "");
 
   ExpectParseFailureForJson(
       "TimestampJsonInputTooSmall", REQUIRED,