Fix failing tests

Author: MaggieKimani1

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiSchemaTests.cs

@@ -337,7 +337,7 @@ public void SerializeV3SchemaWithNullableAsV31Works()
         {
             // Arrange
             var expected = @"type:
-  - null
+  - 'null'
   - string";
 
             var path = Path.Combine(SampleFolderPath, "schemaWithNullable.yaml");
@@ -357,7 +357,7 @@ public void SerializeV2SchemaWithNullableExtensionAsV31Works()
         {
             // Arrange
             var expected = @"type:
-  - null
+  - 'null'
   - string
 x-nullable: true";