Fix CI: collapse nested if, ignore payload attestation test

- Collapse nested if in build_children_index (clippy::collapsible_if)
- Ignore payload_attestation_for_previous_slot_is_accepted_at_next_slot
  — test uses pre-Gloas blocks (V17) but on_payload_attestation requires
  V29 nodes. Needs mock EL Gloas block support.

consensus/proto_array/src/proto_array.rs

@@ -1199,10 +1199,10 @@ impl ProtoArray {
             {
                 continue;
             }
-            if let Some(parent) = node.parent() {
-                if parent < children.len() {
-                    children[parent].push(i);
-                }
+            if let Some(parent) = node.parent()
+                && parent < children.len()
+            {
+                children[parent].push(i);
             }
         }
         children