Can I use N's Zoroark ex's "Trade" Ability if there are no cards in my deck, just to discard a card?
No, you can't use Trade if there aren't any cards left in your deck. Discarding a card from your hand is a cost to use the Ability, and drawing cards is the effect.
Source: Journey Together FAQ; TPCi Rules Team (2025-03-27)

Relevant cards

Note: All applicable cards may not be listed below.
SV9_98
SV9_98