Missing nullability warning for spreads of maybe-null collections

[jnm2]

Version Used: 17.11 preview 6

There should be a warning "CS8602 Dereference of a possibly null reference" in the following example, but there is not:

List<int>? maybeNull = null;
List<int> clone = [.. maybeNull];

When this code runs, it throws NullReferenceException.

In both https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/collection-expressions#known-length-translation and https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/collection-expressions#unknown-length-translation, the translation uses foreach over the expression being spread, which would equivalently throw:

foreach (var _ in maybeNull) // Both warns and throws

[jaredpar]

This is effectively a dupe of #68786