[Bug]: TransformImmutable: For ChangeReason.Add, a previous value cannot be specified

[JakenVeina]

Describe the bug 🐞

The following snippet produces an ArgumentException.

Step to reproduce

using var sc = new SourceCache<string, string>(s => s);
using var sub = sc.Connect().TransformImmutable(s => s.Length).Subscribe();
sc.AddOrUpdate("Hey");

Additional information ℹ️

This is related to a bogus use of default within a ternary expression, here. default here resolves to TDestination instead of Optional<TDestination>, when TDestination is a value type.

[github-actions]

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