The damage Questing Beast deals to the target planeswalker as its last ability resolves isn't combat damage.

Once a creature with power 3 or greater has blocked this creature, changing the power of the blocking creature won't cause this creature to become unblocked.

Questing Beast only stops combat damage from being prevented by effects that specifically use the word “prevent.”

If the opponent dealt damage controls no planeswalkers, Questing Beast's last ability simply does nothing.