Add entity validation to OdometryPublisher

[Bogdanov-am]

🦟 Bug fix

Until SystemManager has the ability to unload system plugins, plugins require an explicit check of the validity of the entities used in the Update methods. Such a check was missing in OdometryPublisher, which led to non-critical but annoying errors in the console.

P.S. It's my first PR to open source project, sorry if i missing something

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 65.95%. Comparing base (633ce72) to head (89a9a54).

:exclamation: Current head 89a9a54 differs from pull request most recent head 50a9b4e. Consider uploading reports for the commit 50a9b4e to get more accurate results

Files	Patch %	Lines
...rc/systems/odometry_publisher/OdometryPublisher.cc	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-sim8 #2326 +/- ## =========================================== - Coverage 65.97% 65.95% -0.02% =========================================== Files 327 327 Lines 31328 31332 +4 =========================================== - Hits 20668 20665 -3 - Misses 10660 10667 +7 ```

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[g-arjones]

@azeey Any idea when this would be released?

[azeey]

We'll shoot for making a release this week.

[g-arjones]

Awesome. Thanks for the heads up :+1: