Fix nullable annotation

Author: eerhardt

Microsoft.NET.Build.Containers/Descriptor.cs

@@ -36,7 +36,7 @@ public readonly record struct Descriptor
     /// <see href="https://github.com/opencontainers/image-spec/blob/7b36cea86235157d78528944cb94c3323ee0905c/descriptor.md#digests"/>
     /// </remarks>
     [JsonIgnore]
-    public string UncompressedDigest { get; init; }
+    public string? UncompressedDigest { get; init; }
 
     /// <summary>
     /// Size, in bytes, of the raw content.