fix: parsing failure on nodes set to null

Author: baywet

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

@@ -33,7 +33,7 @@ public MapNode(ParsingContext context, JsonNode node) : base(
             }
 
             _node = mapNode;
-            _nodes = _node.Select(p => new PropertyNode(Context, p.Key, p.Value)).ToList();
+            _nodes = _node.Where(static p => p.Value is not null).Select(p => new PropertyNode(Context, p.Key, p.Value)).ToList();
         }
 
         public PropertyNode this[string key]
@@ -66,7 +66,7 @@ public override Dictionary<string, T> CreateMap<T>(Func<MapNode, OpenApiDocument
                           : default;
                     }
                     finally
-                    {    
+                    {
                         Context.EndObject();
                     }
                     return new

test/Microsoft.OpenApi.Tests/Reader/MapNodeTests.cs

@@ -0,0 +1,19 @@
+using System.Text.Json.Nodes;
+using Microsoft.OpenApi.Reader;
+using Microsoft.OpenApi.Reader.ParseNodes;
+using Xunit;
+
+namespace Microsoft.OpenApi.Tests.Reader;
+
+public class MapNodeTests
+{
+    [Fact]
+    public void DoesNotFailOnNullValue()
+    {
+        var jsonNode = JsonNode.Parse("{\"key\": null}");
+        var mapNode = new MapNode(new ParsingContext(new()), jsonNode);
+
+        Assert.NotNull(mapNode);
+        Assert.Empty(mapNode);
+    }
+}