Fix dereferenced variables might be null

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/ParseNodes/ListNode.cs

@@ -26,7 +26,7 @@ public override List<T> CreateList<T>(Func<MapNode, T> map)
                 //throw new OpenApiReaderException($"Expected list at line {_nodeList.Start.Line} while parsing {typeof(T).Name}", _nodeList);
             }
 
-            return _nodeList.Select(n => map(new MapNode(Context, n as JsonObject)))
+            return _nodeList?.Select(n => map(new MapNode(Context, n as JsonObject)))
                 .Where(i => i != null)
                 .ToList();
         }

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

@@ -190,7 +190,7 @@ public string GetScalarValue(ValueNode key)
                 //throw new OpenApiReaderException($"Expected scalar at line {_node.Start.Line} for key {key.GetScalarValue()}", Context);
             }
 
-            return scalarNode.ToString();
+            return scalarNode?.GetValue<string>();
         }
 
         /// <summary>

src/Microsoft.OpenApi.Readers/ParseNodes/ValueNode.cs

@@ -22,7 +22,7 @@ public ValueNode(ParsingContext context, JsonNode node) : base(
 
         public override string GetScalarValue()
         {
-            return _node.ToString();
+            return _node.GetValue<string>();
         }
 
         /// <summary>

src/Microsoft.OpenApi.Readers/YamlHelper.cs

@@ -1,12 +1,9 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
-using System.Globalization;
 using System.IO;
 using System.Linq;
-using System.Text.Json;
 using System.Text.Json.Nodes;
-using Microsoft.OpenApi.Exceptions;
 using SharpYaml.Serialization;
 
 namespace Microsoft.OpenApi.Readers
@@ -22,7 +19,7 @@ public static string GetScalarValue(this JsonNode node)
                 //throw new OpenApiException($"Expected scalar at line {node.Start.Line}");
             }
 
-            return scalarNode.ToString();
+            return scalarNode?.GetValue<string>();
         }
 
         public static JsonNode ParseJsonString(string yamlString)