storage proof key - use 0x0 for zero

[macfarla]

PR description

return 0x0 for zero key rather than 0x

Added a test for visibility

Fixed Issue(s)

related to #5377
related to rpc-compat hive tests

this fixes 1 hive test:
eth_getProof/get-account-proof-with-storage

Refs this comment 5cb6c26#r2017642440
This was discussed on RPC standards call and since odd-length hex strings are widely used we will stick with that. Quantity.create handles the zero edge case already.

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests

[macfarla]

Note I had to update several eth_getProof json spec tests to make the key consistent with this PR (remove leading zero from non-zero key). Using Quantity.create makes the format consistent with other places in the output - same as for nonce, the eth_getProof value and other places too.

I don't think this would be a breaking change - and we'll be now consistent with geth - but flagging for reviewers.

[lu-pinto]

LGTM

[macfarla]

@lu-pinto based on your feedback and further chat w Gary, adopted your suggestion to keep the leading zeros where we had them before.