chore: initialize collections to prevent NREs

Author: MaggieKimani1

src/Microsoft.OpenApi/Extensions/OpenApiExtensibleExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 using System;
+using System.Collections.Generic;
 using Microsoft.OpenApi.Exceptions;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Models;
@@ -32,10 +33,8 @@ public static void AddExtension<T>(this T element, string name, IOpenApiExtensio
                 throw new OpenApiException(string.Format(SRResource.ExtensionFieldNameMustBeginWithXDash, name));
             }
 
-            if (element.Extensions is not null)
-            {
-                element.Extensions[name] = Utils.CheckArgumentNull(any);
-            }            
+            element.Extensions ??= new Dictionary<string, IOpenApiExtension>();
+            element.Extensions[name] = Utils.CheckArgumentNull(any);
         }
     }
 }

src/Microsoft.OpenApi/Reader/V31/OpenApiSchemaDeserializer.cs

@@ -10,6 +10,7 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
+using System.Text.Json.Nodes;
 
 namespace Microsoft.OpenApi.Reader.V31
 {
@@ -382,8 +383,9 @@ public static IOpenApiSchema LoadSchema(ParseNode node, OpenApiDocument hostDocu
                 {
                     propertyNode.ParseField(schema, _openApiSchemaFixedFields, _openApiSchemaPatternFields, hostDocument);
                 }
-                else if (schema.UnrecognizedKeywords is not null && propertyNode.JsonNode is not null)
+                else if (propertyNode.JsonNode is not null)
                 {
+                    schema.UnrecognizedKeywords ??= new Dictionary<string, JsonNode>(StringComparer.Ordinal);
                     schema.UnrecognizedKeywords[propertyNode.Name] = propertyNode.JsonNode;
                 }
             }

src/Microsoft.OpenApi/Validations/Rules/OpenApiSchemaRules.cs

@@ -50,7 +50,7 @@ public static bool ValidateChildSchemaAgainstDiscriminator(IOpenApiSchema schema
         {
             if (discriminatorName is not null)
             {
-                if (!schema.Required?.Contains(discriminatorName) ?? false)
+                if (schema.Required is null || !schema.Required.Contains(discriminatorName))
                 {
                     // recursively check nested schema.OneOf, schema.AnyOf or schema.AllOf and their required fields for the discriminator
                     if (schema.OneOf?.Count != 0)