Add Foundry test for failures when spending pre-approved items from null address with invalid signature

[emo-eth]

Test that signature verification works in the case where Seaport has permission to transfer an item at the null address, and fails accordingly.

[0age]

Can we change the phrasing to make it clear that this is testing for failure when attempting this? Only other thing that we might want to tweak is to use the solmate token already available as a library rather than adding the token as a new contract. Let me know if there’s some detail I’m missing though. (Everything else looks solid!)

[emo-eth]

@0age can definitely make language clearer. Solmate tokens don't have virtual modifiers on internal methods, and use the generated public property getters for ie isApprovedForAll, which can't be overridden either. I opted to modify Solmate's 721 rather than add openzeppelin-contracts as a Foundry dependency, but we can do that instead if you'd prefer.