Consider: Why do we have new() as default value for signature and payment config

If you do this on a non-signature task, you will get a new-ed up signature config with all default values.

```csharp
AltinnTaskExtension? taskConfig;
try
{
taskConfig = _processReader.GetAltinnTaskExtension(taskId);
}
catch (Exception)
{
return false;
}

AltinnSignatureConfiguration? signingConfiguration = taskConfig?.SignatureConfiguration;
```

In a non-signature task I would expect it to be null.

Can we remove the default value without causing harm?
```csharp
///
/// Gets or sets the configuration for signature
///
[XmlElement("signatureConfig", Namespace = "http://altinn.no/process")]
public AltinnSignatureConfiguration? SignatureConfiguration { get; set; } = new AltinnSignatureConfiguration();

///
/// Gets or sets the configuration for signature
///
[XmlElement("paymentConfig", Namespace = "http://altinn.no/process")]
public AltinnPaymentConfiguration? PaymentConfiguration { get; set; } = new AltinnPaymentConfiguration();
```

Start with the AltinnTaskExtension, SignatureConfiguration, and PaymentConfiguration declarations shown in the issue, then inspect their XML serialization and deserialization consumers. Confirm whether non-signature and non-payment tasks can leave these properties null without affecting existing configurations; done means the defaults are removed safely and relevant behavior remains covered by existing tests.

Written by the indexing model from the issue text.