[pointer] Update requirements for zero-sized types

Now that [1] is completed, zero-sized accesses no longer require
provenance. Per [2], zero-sized references are no longer required to be
dereferenceable, and so may not carry provenance.

This commit updates `Ptr`'s invariants to not require provenance or a
valid allocation when its referent is zero-sized.

[1] rust-lang/rust#117945
[2] rust-lang/rust#125021

Closes #874

Author: joshlf