for mel, defiant soul can i empower without an enemy unit on a bf

Yes, you can activate the Empower ability on Mel, Defiant Soul even if there is no enemy unit on a battlefield.

According to Rule 827.1.c.1 (and the card text), Mel's Empower ability is functionally: "[Cost]: Empower me. Play only if not Empowered." The cost you must pay is to "Discard a spell." As long as you have a spell in your hand to discard and Mel is not already Empowered, you can activate the ability.

The second part of the ability—"When I become [Empowered], banish an enemy unit at a battlefield with 3 [M] or less"—is a triggered ability that occurs after Mel becomes Empowered. If there is no legal target available when that ability resolves, the instruction will simply fail to find a target and nothing else will happen (as per Rule 359.3.e.6).