wip: improve query memory usage for root field in collect

Author: Yogu

spec/regression/traversal-performance/tests/traversal-performance/aql/RelationAndFieldTraversalWithParent.aql

@@ -12,9 +12,9 @@ RETURN {
       FOR v_node1, v_edge1, v_path1 IN @var3..@var4 OUTBOUND v_super1 @@supers_roots
       RETURN v_node1.`children`[* RETURN {
         "key": CURRENT.`key`,
-        "parent": (IS_NULL(v_node1) ? null : {
+        "parent": {
           "key": v_node1.`key`
-        })
+        }
       }]
     )[**]
   })