improvement: Deprecate empty accounts within framework.

[spencer-tb]

Following the future improvement to deprecate empty accounts -> here, an assertion check has been added to detect empty accounts within the pre and post allocation within tests.

Note that an empty account is one which has zero nonce, code, balance and an empty storage:

"0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b": {
                "nonce": "0x00",
                "balance": "0x00",
                "code": "0x",
                "storage": {}
            }

The beacon root tests have been updated to account for the latter (due to the inclusion of the system address), alongside a few withdrawals tests.

[marioevz]

Looks great!
Just a small change request :)

[marioevz]

LGTM!