Fix rlp for blocks with no withdrawals.

[spencer-tb]

Regarding #36

It looks like before RLP serialization for blocks:

the Block class (block) doesn't set withdrawals to the empty list [ ] for Shanghai, and instead this remains as a None type. This

This PR contains one solution to the latter. As withdrawals are within the Environment class, and it is updated according to the fork type before RLP serialization:

we can use it instead, as the withdrawals None type here is set to the empty list [ ] within set_fork_requirements.

[spencer-tb]

@marioevz

Some other alternative options:

1. To keep the withdrawals field consistent between the Environment class and the Block class we could add block as another input within set_fork_requirements -> set_fork_requirements(env, block, fork), and update the withdrawals for block to [ ] if it has a none type for Shanghai alongside env.
2. Achieve a similar outcome with a new function called set_block_requirements(block, fork).
3. Make the default type for withdrawals in the Block class the empty list as opposed to the None type.

[marioevz]

A couple of things:

• Looking at my code on make_block in BlockchainTests it seems like was really easy to make this mistake in the first place because of how the block and env are just mixed all over the place. I think it would be nice to refactor this a bit on a different PR to make a clear distinction that block structure is the source of the test information, but env is the structure that should contain all the correct information at some point.
• I think this same mistake could be present in the StateTests, it's just that we don't have any withdrawals tests written in the StateTests format.

[marioevz]

LGTM