clean: make QuantizeSettings.CreateInstance parameter non nullable

[Gounlaf]

👋

It seems that this method is only invoked with non-null parameter.

Magick.NET/src/Magick.NET/Native/Settings/QuantizeSettings.cs

Lines 235 to 240 in 32edbce

	internal static INativeInstance CreateInstance(IQuantizeSettings? instance)
	{
	if (instance is null)
	return NativeInstance.Zero;
	return QuantizeSettings.CreateNativeInstance(instance);
	}

This PR remove nullable possibility of parameter.

Regards.

[dlemstra]

This is generated code and I am working on using source generators for this so there is no need to change this code.