add Digest null value to test scenario that was missed

Author: baronfel

test/Microsoft.NET.Build.Containers.IntegrationTests/EndToEndTests.cs

@@ -234,7 +234,7 @@ public async Task TarballsHaveCorrectStructure()
         BuiltImage builtImage = imageBuilder.Build();
 
         // Write the image to disk
-        var sourceReference = new SourceImageReference(registry, DockerRegistryManager.RuntimeBaseImage, DockerRegistryManager.Net7ImageTag);
+        var sourceReference = new SourceImageReference(registry, DockerRegistryManager.RuntimeBaseImage, DockerRegistryManager.Net7ImageTag, null);
         var destinationReference = new DestinationImageReference(new ArchiveFileRegistry(archiveFile), NewImageName(), tags);
 
         return (builtImage, sourceReference, destinationReference);