private V getForNullKey() {
        for (Entry<K, V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }
Offloaded version of get() to look up null keys. Null keys map to index
0. This null case is split out into separate methods for the sake of
performance in the two most commonly used operations (get and put), but
incorporated with conditionals in others.
src/main/java/com/alibaba/fastjson/util/AntiCollisionHashMap.java