优化了平衡，尤其对于向后倾倒的几种极限情况进行优化

Shibo-main-TOMMY/lib/ArduinoJson/extras/tests/JsonDocument/isNull.cpp

@@ -0,0 +1,39 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2023, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("JsonDocument::isNull()") {
+  DynamicJsonDocument doc(4096);
+
+  SECTION("returns true if uninitialized") {
+    REQUIRE(doc.isNull() == true);
+  }
+
+  SECTION("returns false after to<JsonObject>()") {
+    doc.to<JsonObject>();
+    REQUIRE(doc.isNull() == false);
+  }
+
+  SECTION("returns false after to<JsonArray>()") {
+    doc.to<JsonArray>();
+    REQUIRE(doc.isNull() == false);
+  }
+
+  SECTION("returns true after to<JsonVariant>()") {
+    REQUIRE(doc.isNull() == true);
+  }
+
+  SECTION("returns false after set()") {
+    doc.to<JsonVariant>().set(42);
+    REQUIRE(doc.isNull() == false);
+  }
+
+  SECTION("returns true after clear()") {
+    doc.to<JsonObject>();
+    doc.clear();
+    REQUIRE(doc.isNull() == true);
+  }
+}