Clarify EXTCODESIZE of EOF container mid-creation

[chfast]

I'm getting empty contract results from all EXTCODE* opcodes when called to a EOF container mid-creation, which I guess makes sense because the container is not there yet so technically it should still return empty.

Maybe only this should be explicitly specified? @shemnon @chfast @gumb0

Originally posted by @marioevz in ethereum/execution-spec-tests#601 (comment)

[shemnon]

This is how it behaves for legacy. Do we want to change it for EOF? It requires EOF/Legacy calls. And the spec would be a clarification and not a special case - "Like Legacy Initcode, the contract code of an EOF create is empty during an initcall, so EXTCODE* operations from a legacy contract will see zero-length code"

[chfast]

I think we just should clarify that an "EOF container mid-creation" looks like an account without code (i.e. a legacy account).

The alternative to pretend the account has EOF code (i.e. report "EF00" as the code) is no-go because there is no guarantee that the EOF account will be actually created (creation can fail for multiple reasons). So the account may remain empty after creation.