- _from_json detects the "[]" suffix and builds a list of objects, so
  array params (e.g. merge-k-sorted-lists) get a list of ListNode heads
- _from_json returns None for null values instead of crashing
- _to_json serializes None (empty node) as [] to match LeetCode's format
- output serialization distinguishes null nodes ([]) from a missing
  return (null) based on the function's return type