private V putForNullKey(V value) {
        for (Entry<K, V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                return oldValue;
            }
        }
        modCount++;
        addEntry(0, null, value, 0);
        return null;
    }
Offloaded version of put for null keys
src/main/java/com/alibaba/fastjson/util/AntiCollisionHashMap.java