Problem: memiavl result is different when replaying write-ahead-log

[yihuang]

if there are empty values, because protobuf decodes empty value as nil.

Solution:

• make sure empty value is represented as empty slice instead of nil in set operation.

👮🏻👮🏻👮🏻 !!!! REFERENCE THE PROBLEM YOUR ARE SOLVING IN THE PR TITLE AND DESCRIBE YOUR SOLUTION HERE !!!! DO NOT FORGET !!!! 👮🏻👮🏻👮🏻

PR Checklist:

• Have you read the CONTRIBUTING.md?
• Does your PR follow the C4 patch requirements?
• Have you rebased your work on top of the latest master?
• Have you checked your code compiles? (make)
• Have you included tests for any non-trivial functionality?
• Have you checked your code passes the unit tests? (make test)
• Have you checked your code formatting is correct? (go fmt)
• Have you checked your basic code style is fine? (golangci-lint run)
• If you added any dependencies, have you checked they do not contain any known vulnerabilities? (go list -json -m all | nancy sleuth)
• If your changes affect the client infrastructure, have you run the integration test?
• If your changes affect public APIs, does your PR follow the C4 evolution of public contracts?
• If your code changes public APIs, have you incremented the crate version numbers and documented your changes in the CHANGELOG.md?
• If you are contributing for the first time, please read the agreement in CONTRIBUTING.md now and add a comment to this pull request stating that your PR is in accordance with the Developer's Certificate of Origin.

Thank you for your code, it's appreciated! :)

[codecov]

Codecov Report

Merging #1088 (d43ad96) into main (d75ed1a) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1088      +/-   ##
==========================================
- Coverage   46.38%   46.18%   -0.20%     
==========================================
  Files          80       80              
  Lines        7197     7195       -2     
==========================================
- Hits         3338     3323      -15     
- Misses       3500     3516      +16     
+ Partials      359      356       -3     

Impacted Files	Coverage Δ
memiavl/db.go	55.30% <100.00%> (+0.06%)	⬆️
memiavl/tree.go	78.39% <100.00%> (+0.54%)	⬆️

... and 1 file with indirect coverage changes