Rules FAQ
You cannot trigger Yunkai if there are no valid targets; this means you must discard enough gold to remove a character from a challenge.
Essos.
Bestow (4).
Action: During a challenge in which you are the attacking player, kneel Yunkai and discard X gold from it to remove each character with STR X or lower from the challenge (X cannot be 0). Then, if you win the challenge, place 2 gold from the treasury on Yunkai.
Link: Decklists
Rules FAQ
You cannot trigger Yunkai if there are no valid targets; this means you must discard enough gold to remove a character from a challenge.