Fix crash when adding a Light via Component Inspector Editor

[XINJIANGMO]

🦟 Bug fix

Fixes #3131

Summary

When adding a light via Component Inspector Editor, the created entity ends up with a Visual component but no valid Geometry. RenderUtil::CreateVisual assumes Geometry is present and calls visual.SetGeom(_geom->Data()), causing a crash.

This fix return early when _geom is nullptr , avoiding the crash in SetGeom, while not affecting light creation

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)
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• All tests passed (See test coverage)
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[iche033]

nit: this check can be moved to the start of the function, i.e. before sdf::Visual visual

[XINJIANGMO]

Sorry , I do some tests after this fix , but when I rerun the simulation after I add a light to a model , it crashes again.
This is the crash.log.
From the stack , When we rerun the simulation, EntityComponentManager will call nameComp->Data() without checking if it exists.
There may be a better solution for this issue！

[XINJIANGMO]

Also, I find that when I use add function via Component Inspector Editor without pausing, it will not pop up Tip Dialog but stuck.
I test it in gz-sim9 and it works well.

If developers confirm these , I can try my best to find and fix it.