Prevent AvailabilityCheckError when there's no new custody columns to import (#7533)

Addresses a regression recently introduced when we started gossip verifying data columns from EL blobs

```
failures:
network_beacon_processor::tests::accept_processed_gossip_data_columns_without_import

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 90 filtered out; finished in 16.60s

stderr ───

thread 'network_beacon_processor::tests::accept_processed_gossip_data_columns_without_import' panicked at beacon_node/network/src/network_beacon_processor/tests.rs:829:10:
should put data columns into availability cache: Unexpected("empty columns")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

https://github.com/sigp/lighthouse/actions/runs/15309278812/job/43082341868?pr=7521

If an empty `Vec` is passed to the DA checker, it causes an unexpected error.

This PR addresses it by not passing an empty `Vec` for processing, and not spawning a task to publish.

beacon_node/beacon_chain/src/fetch_blobs/tests.rs

@@ -1,4 +1,4 @@
-use crate::data_column_verification::GossipVerifiedDataColumn;
+use crate::data_column_verification::{GossipDataColumnError, GossipVerifiedDataColumn};
 use crate::fetch_blobs::fetch_blobs_beacon_adapter::MockFetchBlobsBeaconAdapter;
 use crate::fetch_blobs::{
     fetch_and_process_engine_blobs_inner, EngineGetBlobsOutput, FetchEngineBlobError,
@@ -139,6 +139,52 @@ async fn test_fetch_blobs_v2_block_imported_after_el_response() {
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_fetch_blobs_v2_no_new_columns_to_import() {
+    let mut mock_adapter = mock_beacon_adapter();
+    let (publish_fn, publish_fn_args) = mock_publish_fn();
+    let (block, blobs_and_proofs) = create_test_block_and_blobs(&mock_adapter);
+    let block_root = block.canonical_root();
+
+    // **GIVEN**:
+    // All blobs returned
+    mock_get_blobs_v2_response(&mut mock_adapter, Some(blobs_and_proofs));
+    // block not yet imported into fork choice
+    mock_fork_choice_contains_block(&mut mock_adapter, vec![]);
+    // All data columns already seen on gossip
+    mock_adapter
+        .expect_verify_data_column_for_gossip()
+        .returning(|c| {
+            Err(GossipDataColumnError::PriorKnown {
+                proposer: c.block_proposer_index(),
+                slot: c.slot(),
+                index: c.index,
+            })
+        });
+    // No blobs should be processed
+    mock_adapter.expect_process_engine_blobs().times(0);
+
+    // **WHEN**: Trigger `fetch_blobs` on the block
+    let custody_columns = hashset![0, 1, 2];
+    let processing_status = fetch_and_process_engine_blobs_inner(
+        mock_adapter,
+        block_root,
+        block,
+        custody_columns.clone(),
+        publish_fn,
+    )
+    .await
+    .expect("fetch blobs should succeed");
+
+    // **THEN**: Should NOT be processed and no columns should be published.
+    assert_eq!(processing_status, None);
+    assert_eq!(
+        publish_fn_args.lock().unwrap().len(),
+        0,
+        "no columns should be published"
+    );
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_fetch_blobs_v2_success() {
     let mut mock_adapter = mock_beacon_adapter();