RLP encoding of 0 (zero) as 0x80

[pinges]

Description

I have created a PR to fix a problem with Nethermind clients (#4283), where Besu cannot decode the Hello message sent by Nethermind because it contains a capability for the witness protocol with version 0. This 0 was encoded 0x80, which is the encoding for a zero length string but in this case needs to be interpreted as the integer value 0.
I learned that this is (probably) the right way to encode the value 0.

The following should be done:

• research where 0x80 is the right way to encode the value 0 and, if necessary, fix the RLP encoding in Besu
• fix RLP decoding where necessary

[pinges]

https://ethereum.org/en/developers/docs/data-structures-and-encoding/rlp/ :
"Deserialized positive integers with leading zeroes get treated as invalid. The integer representation of string length must also be encoded this way, as well as integers in the payload."

[macfarla]

Ethereum yellow paper spells out the removal of all leading zeros in appendix B just below equation 197, and that zero scaler writes as 0x80 is explicitly tested for in the ethereum reference tests. and a number of invalid tests flag 0x00 as a scalar length as invalid.

[shemnon]

The above was from the original bug - #4279 (comment)

[pinges]

In the WireMessagesSedesTest we are currently disabling a check because we are encoding the version number zero as 0x00, not 0x80. As part of the ticket that check might be enabled again.