Problem: empty topics shouldn't be encoded as nil

[yihuang]

Closes: #839

Description

Solution:

• encode it as empty array

---

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)

[codecov]

Codecov Report

Merging #840 (0cd047b) into main (423944b) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #840   +/-   ##
=======================================
  Coverage   57.31%   57.31%           
=======================================
  Files          72       72           
  Lines        5984     5984           
=======================================
  Hits         3430     3430           
  Misses       2357     2357           
  Partials      197      197           

Impacted Files	Coverage Δ
x/evm/types/logs.go	30.58% <0.00%> (ø)

[fedekunze]

ACK, can you add a changelog entry?

[yihuang]

ACK, can you add a changelog entry?

done