Does Loyal Pub work on empty battlefields? So can I trigger his when you defend when opponent moves to an open bf?

No, Loyal Pup's ability does not trigger when an opponent moves to an empty battlefield.

According to Rule 446.1 and RiftJudge FAQ #8664, when a unit moves to an empty (unoccupied) battlefield, it creates a non-combat Showdown, not Combat.

For a unit to "defend" (and thus trigger "When you defend" abilities), there must be an active Combat at that battlefield. Combat only occurs when units controlled by opposing players are present at the same battlefield (Rule 447.1). Since moving to an empty battlefield initiates a non-combat showdown rather than combat, you do not gain the Defender designation, and therefore "When you defend" triggers do not occur.