fix(hql): handle TraversalValue::Empty in DROP operations (#673)

## Summary

This PR integrates the fix from #670 into the `arena-implementation`
branch. It resolves runtime errors when attempting to DROP non-existent
nodes, edges, or vectors.

## Problem

Queries attempting to drop non-existent items failed with:
```
Conversion error: Incorrect Type: Empty
```

This occurred because `drop_traversal` did not handle
`TraversalValue::Empty` returned when querying for non-existent items.

## Solution

Added `TraversalValue::Empty => Ok(())` case in the match statement of
`drop_traversal` in
`helix-db/src/helix_engine/traversal_core/ops/util/drop.rs:51`.

This treats empty traversals as successful no-ops, making DROP
operations idempotent.

## Testing

- ✅ `cargo check` passes with no errors
- ✅ All 109 `helix_engine` tests pass, including all DROP-related tests

## Related

- Integrates fix from PR #670
- No changes needed to helixc generator - the fix automatically applies
to generated code

Co-Authored-By: ishaksebsib

Author: xav-db

helix-db/src/helix_engine/traversal_core/ops/util/drop.rs

@@ -48,6 +48,7 @@ where
                             Err(e) => Err(e),
                         }
                     }
+                    TraversalValue::Empty => Ok(()),
                     _ => Err(GraphError::ConversionError(format!(
                         "Incorrect Type: {item:?}"
                     ))),