chore: avoid systematically cloning to reduce performance impact

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi.YamlReader/YamlConverter.cs

@@ -124,7 +124,7 @@ private static JsonValue ToJsonValue(this YamlScalarNode yaml)
             {
                 ScalarStyle.Plain when decimal.TryParse(yaml.Value, NumberStyles.Float, CultureInfo.InvariantCulture, out var d) => JsonValue.Create(d),
                 ScalarStyle.Plain when bool.TryParse(yaml.Value, out var b) => JsonValue.Create(b),
-                ScalarStyle.Plain when YamlNullRepresentations.Contains(yaml.Value) => JsonNullSentinel.JsonNull,
+                ScalarStyle.Plain when YamlNullRepresentations.Contains(yaml.Value) => (JsonValue)JsonNullSentinel.JsonNull.DeepClone(),
                 ScalarStyle.Plain => JsonValue.Create(yaml.Value),
                 ScalarStyle.SingleQuoted or ScalarStyle.DoubleQuoted or ScalarStyle.Literal or ScalarStyle.Folded or ScalarStyle.Any => JsonValue.Create(yaml.Value),
                 _ => throw new ArgumentOutOfRangeException(nameof(yaml)),

src/Microsoft.OpenApi/JsonNullSentinel.cs

@@ -20,10 +20,10 @@ public static class JsonNullSentinel
     /// This can only be used for OpenAPI properties of type <see cref="JsonNode"/>.
     /// This can only be used for the root level of a JSON structure.
     /// Any use outside of these constraints is unsupported and may lead to unexpected behavior.
-    /// Because this is returning a cloned instance, so the value can be added in a tree, reference equality checks will not work.
+    /// Because the value might be cloned, so the value can be added in a tree, reference equality checks will not work.
     /// You must use the <see cref="IsJsonNullSentinel(JsonNode?)"/> method to check for this sentinel.
 	/// </summary>
-    public static JsonValue JsonNull => (JsonValue)SentinelJsonValue.DeepClone();
+    public static JsonValue JsonNull => SentinelJsonValue;
 
     /// <summary>
     /// Determines if the given node is the JSON null sentinel.
@@ -32,7 +32,8 @@ public static class JsonNullSentinel
     /// <returns>Whether or not the given node is the JSON null sentinel.</returns>
     public static bool IsJsonNullSentinel(this JsonNode? node)
     {
-        return node is JsonValue jsonValue &&
+        return node == SentinelJsonValue ||
+                node is JsonValue jsonValue &&
                 jsonValue.GetValueKind() == JsonValueKind.String &&
                 jsonValue.TryGetValue<string>(out var value) &&
                 SentinelValue.Equals(value, StringComparison.Ordinal);