Get item in cache for cache_key and add item if item is not None.

def update_cache_key(cls, cache_key, item=None):
        """Get item in cache for cache_key and add item if item is not None.
        """
        contents = cls.__thread_id_cache.get(cache_key, None)
        if item is not None:
            cls.__thread_id_cache[cache_key] = item

        return contents