chore(tvix/store): use DUMMY_DIGEST, too

Missed from cl/7649.

Change-Id: Id2d382567f91cba9cdd792fe44bc0a83d040ec69
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7656
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/proto.rs

@@ -43,7 +43,7 @@ mod tests {
             let d = Directory {
                 directories: vec![DirectoryNode {
                     name: String::from("foo"),
-                    digest: vec![],
+                    digest: DUMMY_DIGEST.to_vec(),
                     size: 0,
                 }],
                 ..Default::default()