Clean up tests and add a null check

Author: MaggieKimani1

src/Microsoft.OpenApi/Writers/OpenApiWriterAnyExtensions.cs

@@ -32,7 +32,15 @@ public static void WriteExtensions(this IOpenApiWriter writer, IDictionary<strin
                 foreach (var item in extensions)
                 {
                     writer.WritePropertyName(item.Key);
-                    item.Value.Write(writer, specVersion);
+                    
+                    if (item.Value == null)
+                    {                        
+                        writer.WriteNull();
+                    }
+                    else
+                    {
+                        item.Value.Write(writer, specVersion);
+                    }
                 }
             }
         }

test/Microsoft.OpenApi.Tests/Models/OpenApiTagTests.cs

@@ -168,7 +168,7 @@ public void SerializeAdvancedTagAsV3YamlWithoutReferenceWorks()
 externalDocs:
   description: Find more info here
   url: https://example.com
-x-tag-extension:";
+x-tag-extension: ";
 
             // Act
             AdvancedTag.SerializeAsV3WithoutReference(writer);
@@ -193,7 +193,7 @@ public void SerializeAdvancedTagAsV2YamlWithoutReferenceWorks()
 externalDocs:
   description: Find more info here
   url: https://example.com
-x-tag-extension:";
+x-tag-extension: ";
 
             // Act
             AdvancedTag.SerializeAsV2WithoutReference(writer);