Do not spawn validator backwards pass if count is zero (#2315)

### Description

Fixes a bug in which the validator would panic if the mailbox was empty.

### Drive-by changes

None

rust/agents/validator/src/validator.rs

@@ -152,13 +152,13 @@ impl Validator {
             .tree(lag)
             .await
             .expect("failed to get mailbox tree");
-        let backfill_target = submitter.checkpoint(&tip_tree);
-
         let mut tasks = vec![];
 
-        let backfill_submitter = submitter.clone();
         let legacy_submitter = submitter.clone();
 
+        if tip_tree.count() > 0 {
+            let backfill_target = submitter.checkpoint(&tip_tree);
+            let backfill_submitter = submitter.clone();
             tasks.push(
                 tokio::spawn(async move {
                     backfill_submitter
@@ -167,6 +167,8 @@ impl Validator {
                 })
                 .instrument(info_span!("BackfillCheckpointSubmitter")),
             );
+        }
+
         tasks.push(
             tokio::spawn(async move { submitter.checkpoint_submitter(tip_tree, None).await })
                 .instrument(info_span!("TipCheckpointSubmitter")),