Can you use LeBlanc legend without a card to discard?

No, you cannot activate the Deceiver legend's ability if you have no cards in your hand.

According to Rule 422.3, when discarding is listed as a cost, the action must be able to be completed for the cost to be paid. Because the text of the Deceiver legend uses "discard 1" as a cost to activate the ability, you must have at least one card in your hand to discard in order to successfully activate it.

If you have no cards in your hand, you cannot pay the cost, and therefore you cannot activate the ability.