Chronologically "Null" happens between the events of Happiness 2 and A Happiness Story.
But CC doesn't go through games chronologically. She goes by order of release so to her Null happened after Goodbye time traveler and A happiness Story.
As for you the player you can just follow the playing order on my creator page https://fouzi94.itch.io/
And you'll be fine.
Thank you for playing the series . I appreciate it