QIf my follower is being attacked by an enemy follower, but is destroyed by a card or ability such as Alucard's Strike ability, Dark Offering, or Soul Conversion, what happens to the attacking follower's attack?

AIf an attack target leaves the field during a follower attack, a new target is not selected, the attacking follower remains engaged, and combat damage is not dealt. Furthermore, even if the attacking follower or attack target leaves the field during the attack, cards and abilities with Quick can still be played.

QIf there are no enemy followers on the field, does this follower's Strike ability still give my leader +4 defense?

ANo. You must be able to select an enemy follower to play the Strike ability.