Fix QML warnings from Inertial.qml in Component Inspector

[iche033]

🦟 Bug fix

Summary

Fixes a flood of warnings when clicking on a link of a model in the Entity Tree GUI plugin:

(2025-03-21 18:23:44.927) [warning] [Application.cc:915] [GUI] [QT] file::/ComponentInspector/Inertial.qml:343:13: Unable to assign [undefined] to QString
(2025-03-21 18:23:44.927) [warning] [Application.cc:915] [GUI] [QT] file::/ComponentInspector/Inertial.qml:351:13: Unable to assign [undefined] to QString
(2025-03-21 18:23:44.927) [warning] [Application.cc:915] [GUI] [QT] file::/ComponentInspector/Inertial.qml:359:13: Unable to assign [undefined] to QString
...

This is because these extra inertial data only exist if the link has fluid added mass specified

To test:

1. launch shapes.sdf world,
2. Go the the Entity Tree GUI plugin on the right and expand the box model
3. Click on link-> you should no longer see these warnings above

You can also verify that the component inspector's inertial component still works for a link with added mass

1. launch fluid_added_mass.sdf wrold
2. Go the the Entity Tree GUI plugin on the right and expand the box_xx_xy_xz model
3. Click on link-> no warnings should be printed
4. Go to the Component Inspector GUI plugin above the Entity Tree, expand Inertial entry to see the Fluid added mass values.

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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