Add parameterless constructor to StreamResponse (#111)

This is required to fulfill the `new()` constraint on e.g.
`ResponseBuilder.ToResponse()`.

src/Elastic.Transport/Responses/Special/StreamResponse.cs

@@ -24,6 +24,13 @@ public sealed class StreamResponse :
 	/// </summary>
 	public string MimeType { get; }
 
+	/// <inheritdoc cref="StreamResponse"/>
+	public StreamResponse()
+	{
+		Body = Stream.Null;
+		MimeType = string.Empty;
+	}
+
 	/// <inheritdoc cref="StreamResponse"/>
 	public StreamResponse(Stream body, string? mimeType)
 	{