fix(abci): Add serde `default` for `Event::type`

[penso]

Fixes https://github.com/informalsystems/tendermint-rs/issues/1415

• [X] Referenced an issue explaining the need for the change
• [X] Updated all relevant documentation in docs
• [X] Updated all code comments where relevant
• [X] Wrote tests
• [x] Added entry in .changelog/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.22222% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 59.5%. Comparing base (99ed0b9) to head (90ddb0a). Report is 10 commits behind head on main.

:exclamation: Current head 90ddb0a differs from pull request most recent head 775d001. Consider uploading reports for the commit 775d001 to get more accurate results

Files	Patch %	Lines
rpc/tests/dydx_fixtures.rs	97.2%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1416 +/- ## ======================================= - Coverage 60.1% 59.5% -0.6% ======================================= Files 271 272 +1 Lines 26221 26795 +574 ======================================= + Hits 15768 15963 +195 - Misses 10453 10832 +379 ```

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