Add more tests for JsonVariantConst

extras/tests/JsonVariantConst/isnull.cpp

@@ -0,0 +1,21 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2024, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("JsonVariantConst::isNull()") {
+  JsonDocument doc;
+  JsonVariantConst variant = doc.to<JsonVariant>();
+
+  SECTION("returns true when undefined") {
+    REQUIRE(variant.isNull() == true);
+  }
+
+  SECTION("returns false if value is integer") {
+    doc.set(42);
+
+    REQUIRE(variant.isNull() == false);
+  }
+}