Does the healing from the Wither and Bloom spell only happen if the target has at least one unspent Hit Die?

Question

The wither and bloom spell reads, in part (Strixhaven: A Curriculum of Chaos, p. 38):

[...] one creature of your choice in that area can spend and roll one of its unspent Hit Dice and regain a number of hit points equal to the roll plus your spellcasting ability modifier.

Does this mean the healing from the wither and bloom spell can only happen if the creature has at least one unspent Hit Die?

Or does the spell still heal a number of hit points equal to the spellcasting modifier, even if the creature has no unspent Hit Dice?

Answer 1

No healing happens

Spells only do what they say, so if the target has no unspent Hit Dice, it will not be able to heal by spending (the non-existing) unspent Hit Dice.

Answer 2

The healing is contingent on having an unspent Hit Die

The healing only occurs if the creature spends one of their Hit Dice, if they do not have any available, they do not heal.

Answer 3

The spell is still cast (and expended), but since the player cannot spend Hit Dice (they have none left), no healing occurs. The modifier is just that... a modifier. Since no Hit Dice are available to expend for healing, there is nothing to modify, so it is wasted (though the other affects of necrotic damage and withering vegetation are still very much applied).