Can I cast Void Assault if I don't want to move a friendly unit, only an enemy unit?

No, you cannot cast Void Assault if you do not want to move a friendly unit.

According to the card's text, the ability is: "Move a friendly unit, then move an enemy unit." In Riftbound, when a spell or ability contains instructions connected by "then," those actions are mandatory steps of the effect. Because the spell requires you to move a friendly unit as the first part of its instruction, you must be able to perform that action to initiate the spell.

If you have no friendly units that you are able or willing to move, you cannot fulfill the requirements of the card, and therefore cannot cast it.