TZIP-5: What should happen in FA1 transfer if the from account does not exist?

What should happen in the entrypoint transfer if the source of the transfer has no account in the ledger. The specification states:

In case of insufficient balance, this entrypoint must fail with the following error: ...

However, what is the balance of a non-existing account? following the spec of getBalance, we could say it is zero. But then, what happens if we attempt to transfer 0 tokens from a non-existing account?

Also, on a completely unrelated note: if TZIP-5 (FA1) is deprecated, how about merging it into TZIP-7? There would be no change in semantics, but in this way you wouldn't have to look up both specifications all the time.