I just don't want anything to be left without a loot table because I tried to make a adventure map before the villager loot table was added and I couldn't make them drop what i wanted so it was very frustrating and I'm sure that someone will want to change the wither's loot table for an adventure map and not want to have to manually add the loot table to it using a data tag
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Attached the missing file.
Still an issue in 18w14b.

Just a note that the nether star item entity the wither drops is set to never despawn. It's not quite as straight-forward to switch this process over to loot tables alone, since loot tables return just the item data rather than the item entity.
Also not destroyed by explosions (at least from wither skulls).
They could add a function for setting these kind of properties though.

The property for being explosion-proof is actually an innate property for the instance of a nether star item, not specifically to do with the wither's drops. Any nether star dropped will be explosion-proof (such that it doesn't have to be something new for loot tables, would be more for data-driven items in the future).

Affects 1.13-pre8. Please keep your issue updated!

Affects 18w32a and 1.13.
You could use command blocks to detect when the player holds a nether star and replace it with what you want the wither to drop.