The value of X is determined only once, as Kin-Tree Invocation resolves. The power and toughness of the Spirit Warrior token doesn’t later change if the greatest toughness among creatures you control changes.

If you control no creatures as Kin-Tree Invocation resolves, you’ll put a 0/0 token onto the battlefield. Unless something else is raising its toughness above 0, it will immediately be put into its owner’s graveyard and then cease to exist. Any abilities that trigger when a creature enters the battlefield or dies will trigger.