EVM/node: state override that specifies with an empty storage

What

Follows !14921 (merged)

Part of #7476 (closed)

Implements state part of the state override of eth_call even with an empty state.

see go-ethereum doc for spec.

An example in the doc seems to imply the RPC can accept a state override parameter with an empty state for an address :

{
  "0xebe8efa441b9302a0d7eaecc277c09d20d684540": {
    "code": "0x...",
    "state": {
      ""
    }
  }
}

This particular JSON is not a valid JSON, but the following is:

{
  "0xebe8efa441b9302a0d7eaecc277c09d20d684540": {
    "code": "0x...",
    "state": {}
  }
}

So we make it work.

Why

Because maybe someone need it ? And it's a convenient way to reset the storage of an account ? And it's in the spec, sorta.

How

So now state_override.state is of type hex StorageMap.t option.

We tweak the encoding to map:

• state is not specified -> None
• state is specified but empty -> Some StorageMap.empty
• state is specified and not empty -> same as before

We can't do the obvious encoding Data_encoding.option <map encoding> (or use opt) because both would be nullable, and Data_encoding crashes at run time. ( see Data_encoding doc)

Manually testing the MR

We add a test:

dune exec etherlink/tezt/tests/main.exe -- alpha state_empty

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR