Fix dummy payloads for withdrawals

[Nashtare]

Fixes an oversight from my review of #4.

The issue is that dummy payloads were still being constructed as if they were at the start of a block, even the one possibly created to include the withdrawals vector. This had the consequence that:

• txn and receipt tries were set to EMPTY_TRIE_HASH (and so were their trie_after_roots), while if we create a payload for withdrawals after existing transactions, these tries would not be empty;
• the rest of the GenerationInputs, namely what's commonly called as ExtraBlockData on the plonky2 side was set to Default::default(), which would also cause discrepancies if we were to add a dummy payload after (i.e. gas_used, txn_number, etc).

The fix is kinda drowning in additional cleanup changes (what I was originally doing when I noticed the issue, apologies for this 🙏🏼).

[BGluth]

LGTM!