What’s a Shadow Weapon’s hardness?

The first-level spell shadow weapon creates an illusionary weapon the caster can wield. Consequently, the weapon thereby produced may find itself subjected to all manner of hostile effects by enemies who believe the weapon to be real (those who don’t aren’t usually too scared of the 1 damage it might inflict). It’s unclear from the spell description, however, what the hardness and hp should be. How does one calculate that, and what are the values?