chore(test-types): Rename `pre.empty_account`

I'm now thinking that this `empty_account` name could be misleading. We are not placing an account with all fields set to zero in the pre-state, as one might expect an empty account to be, rather the returned address is an address that is nowhere in the pre-state.

Perhaps `pre.nonexistent_address()` is a better name.

I don't think we currently can put an actual empty account in the pre-state, but after looking at the original static tests, it doesn't try to put an empty account either, so it might be ok?

_Originally posted by @marioevz in https://github.com/ethereum/execution-specs/pull/2388#discussion_r2892974099_

Nice-to-have to rename `empty_account` to something that better describes what this method does, and reserve `empty_account` for the future if we at any point are able to create empty accounts in some of the forks.