chore: refactoring

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

Author: baywet

src/Microsoft.OpenApi/Reader/ParseNodes/MapNode.cs

@@ -41,9 +41,9 @@ public PropertyNode? this[string key]
         {
             get
             {
-                if (_node.ContainsKey(key))
+                if (_nodes.TryGetValue(key, out var value))
 				{
-					return _nodes[key];
+					return value;
 				}
 
                 return null;