Added support for nullptr (closes #998)

test/MixedConfiguration/cpp11.cpp

@@ -0,0 +1,31 @@
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+
+#if __cplusplus >= 201103L
+
+TEST_CASE("nullptr") {
+  DynamicJsonDocument doc(4096);
+  JsonVariant variant = doc.to<JsonVariant>();
+
+  SECTION("JsonVariant == nullptr") {
+    REQUIRE((variant == nullptr));
+    REQUIRE_FALSE((variant != nullptr));
+  }
+
+  SECTION("JsonVariant != nullptr") {
+    variant.set(42);
+
+    REQUIRE_FALSE((variant == nullptr));
+    REQUIRE((variant != nullptr));
+  }
+
+  SECTION("JsonVariant.set(nullptr)") {
+    variant.set(42);
+    variant.set(nullptr);
+
+    REQUIRE(variant.isNull());
+  }
+}
+
+#endif