Fix RodModelToMjcf.convert when no cameras argument is passed

[diegoferigo]

Regression introduced in #133.

---

📚 Documentation preview 📚: https://jaxsim--135.org.readthedocs.build//135/

[flferretti]

Thanks! I wasn't expecting that issue as in theory cameras should be converted to a list of an empty dictionary. We should probably add some tests on the visualizer for the next release

[diegoferigo]

Thanks! I wasn't expecting that issue as in theory cameras should be converted to a list of an empty dictionary. We should probably add some tests on the visualizer for the next release

The problem is that a list of empty dictionaries like [{}] enters the for loop and triggers the construction of a MujocoCamera instance. This PR ensures that no MujocoCamera is created when cameras=None.