Prevent printing misleading error msg in visualize lidar gui plugin

[iche033]

🦟 Bug fix

Prevents the following msg from being printed when the scene and plugin are loaded on startup:

[GUI] [Err] [VisualizeLidar.cc:252] Lidar pointer is not set

Related question:
https://robotics.stackexchange.com/questions/25151/gazebo-garden-gui-err-visualizelidar-cc251-lidar-pointer-is-not-set

Summary

Currently we don't create the lidar visual if the scene does not have any visuals (happens on startup). As the result, in the first render event, the error msg is printed. This does not need to be the case. It is safe to create the lidar visual even if the scene is empty.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #2077 (3398cba) into gz-sim7 (579013f) will not change coverage.
The diff coverage is n/a.

❗ Current head 3398cba differs from pull request most recent head de852dc. Consider uploading reports for the commit de852dc to get more accurate results

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  Hits         18702    18702           
  Misses       10030    10030