What is the set containing no elements called?

Empty set

What's the name of Link's primary sword in the 'Legend of Zelda' series? Which Pokémon move absorbs half of the damage dealt as health back to the user? What traditionally circular Indian board game involves flicking discs called carrom men into corner pockets? What condiment is traditionally served with roast beef sandwiches, especially in the UK? What mobile base does the X-Men team originally use as an aircraft?