@Ivan Petrov answer did the job, also his answer made me thing using options pattern with default values for fields which looks like this:

public class PVO_HttpClientBasicConfig
{
    public string HttpClientName { get; set; } = "PVO_HttpClient";
...
}

So if there is no key of HttpClientName in appsettgins.json it will use the default value from class definition -> this won't work for null values in appsettings.json