Missing nullability warnings for semi-autoproperties

[TessenR]

Version Used:

Branch main (17 Dec 2024)
Latest commit [f9ccce0](https://github.com/dotnet/roslyn/commit/f9ccce030d8fdcc2ea095e095b71316d243c5b35) by Phil Allen:
Update PublishData.json (#76471)

Steps to Reproduce:

Compile and run the following code (sharplab.io)

using System;
using System.Diagnostics.CodeAnalysis;
#nullable enable

var p = new C().Prop;
Console.WriteLine(p.Length);

class C
{
  [NotNull]
  public string? Prop
  {
    get;
    set => field = value ?? string.Empty;
  } = null;
}

Expected Behavior:
A warning is emitted for get; accessor that returns a possibly nullable field value as a result of a non-nullable property

Actual Behavior:
No warnings at all in the program above. It crashes with a NullReferenceException at runtime

Notes:
If you explicitly implement get; with the equivalent get => field; you get the correct CS8603: Possible null reference return. warning