Nullability warnings are not reported inside collection expressions

[thinker227]

Version Used: 4.8.0-1.23374.10 (520e2f1)

Steps to Reproduce:

1. Assign a collection expression containing a null literal (or some other value which may be null) to a collection type which element type is not nullable.

IEnumerable<string> xs = ["a", null, "b"];

Sharplab link

Expected Behavior: CS8625 (or another applicable nullability warning) is reported on null.

Relates to #66418

Actual Behavior: No nullability warnings are reported.

[thinker227]

Was informed this is already being worked on.

[CyrusNajmabadi]

Reactivating while the issue still repros

[jcouv]

Closing as duplicate of #68786