eth_feeHistory reward values cannot be nil

[loredanacirstea]

Closes: #XXX

Description

Querying the eth_feeHistory can return a reward attribute with null values, as seen with:

curl https://eth.bd.evmos.org:8545 \
    -X POST \
    -H "Content-Type: application/json" \
    -d '{"jsonrpc":"2.0","method":"eth_feeHistory","params":["0x4", "0x3a6e", [25, 75]],"id":1}'

{"jsonrpc":"2.0","method":"eth_feeHistory","params":["0x4", "0x3a6e", [25, 75]],"id":1}'
{"jsonrpc":"2.0","id":1,"result":{"oldestBlock":"0x3a6a","reward":[[null,null],[null,null],[null,null],[null,null]],"baseFeePerGas":["0x7","0x7","0x7","0x7"],"gasUsedRatio":[0.0034889,0.01266025,0.019304325,0.0153471]}}

Even if the block does not have transactions, the value should always be 0x0.
The reward attribute only exists if rewardPercentiles is given as input.

---

For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

---

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[loredanacirstea]

This is a draft PR now, because I could not reproduce it in a test. I tried modifying the TestEth_FeeHistory test in rpc_test.go, checking the following cases:

• block with no transactions
• blocks with a negative GasTipCap

But in none of these cases was a nil value returned.

[fedekunze]

ACK, pending bugfix changelog entry