iOS only: security_framework::passwords::get_password_and_release crashes on empty password

[brotskydotcom]

On Rust 1.79.0, line 163 in passwords.rs

vec.extend_from_slice(val.bytes());

crashes if val is empty, with the message:

unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed isize::MAX

This can easily be reproduced if the existing generic_roundtrip test is modified to use an empty password string.

This bug doesn't exist on macOS, because the macOS-specific implementation doesn't retrieve data using this function.