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

This fix resolves runtime errors when attempting to DROP non-existent
nodes, edges, or vectors. Previously, empty traversals would throw
"Conversion error: Incorrect Type: Empty" errors.

Added TraversalValue::Empty => Ok(()) case to make DROP operations
idempotent - dropping non-existent items now succeeds silently.

Integrates PR #670 into the arena-implementation branch.

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:?}"
                     ))),