Fix nullability annotations in SystemTextJsonSerializer (#123)

Fix nullability annotations in `SystemTextJsonSerializer`.

src/Elastic.Transport/Components/Serialization/SystemTextJsonSerializer.cs

@@ -3,10 +3,8 @@
 // See the LICENSE file in the project root for more information
 
 using System;
-using System.Collections.Generic;
 using System.IO;
 using System.Text.Json;
-using System.Text.Json.Serialization;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -18,8 +16,8 @@ namespace Elastic.Transport;
 /// </summary>
 public abstract class SystemTextJsonSerializer : Serializer
 {
-	private readonly JsonSerializerOptions? _options;
-	private readonly JsonSerializerOptions? _indentedOptions;
+	private readonly JsonSerializerOptions _options;
+	private readonly JsonSerializerOptions _indentedOptions;
 
 	/// <summary>
 	/// An abstract implementation of a transport <see cref="Serializer"/> which serializes using the Microsoft
@@ -90,8 +88,8 @@ public override Task SerializeAsync<T>(T data, Stream stream,
 	/// Returns the <see cref="JsonSerializerOptions"/> for this serializer, based on the given <paramref name="formatting"/>.
 	/// </summary>
 	/// <param name="formatting">The serialization formatting.</param>
-	/// <returns>The requested <see cref="JsonSerializerOptions"/> or <c>null</c>, if the serializer is not initialized yet.</returns>
-	protected internal JsonSerializerOptions? GetJsonSerializerOptions(SerializationFormatting formatting = SerializationFormatting.None) =>
+	/// <returns>The requested <see cref="JsonSerializerOptions"/>.</returns>
+	protected internal JsonSerializerOptions GetJsonSerializerOptions(SerializationFormatting formatting = SerializationFormatting.None) =>
 		formatting is SerializationFormatting.None ? _options : _indentedOptions;
 
 	private static bool TryReturnDefault<T>(Stream? stream, out T deserialize)

src/Elastic.Transport/Components/Serialization/TransportSerializerExtensions.cs

@@ -185,8 +185,6 @@ public static string SerializeToString(
 		return ms.Utf8String();
 	}
 
-	#region STJ Extensions
-
 	/// <summary>
 	/// Extension method that writes the serialized representation of an instance of <typeparamref name="T"/> to a
 	/// <see cref="Utf8JsonWriter"/>.
@@ -633,6 +631,4 @@ public static void Serialize(
 
 		return serializer.Deserialize(type, ms);
 	}
-
-	#endregion STJ Extensions
 }