Allow building a MJCF without user-defined cameras

[flferretti]

This PR fixes a bug for which the MJCF could not be built without passing a user-defined camera.

Testing Script

import rod
from rod.builder.primitives import SphereBuilder

from jaxsim.mujoco import UrdfToMjcf

model_sdf_string = rod.Sdf(
    version="1.7",
    model=SphereBuilder(radius=0.10, mass=1.0, name="sphere")
    .build_model()
    .add_link()
    .add_inertial()
    .add_visual()
    .add_collision()
    .build(),
).serialize(pretty=True)

mjcf_string, assets = UrdfToMjcf.convert(
    urdf=model_sdf_string,
)

Error Log

File ~/jaxsim/src/jaxsim/mujoco/loaders.py:496, in RodModelToMjcf.convert(rod_model, considered_joints, plane_normal, heightmap, heightmap_samples_xy, cameras)
    490 # Add user-defined camera.
    491 for camera in cameras if isinstance(cameras, Sequence) else [cameras]:
    493     mj_camera = (
    494         camera
    495         if isinstance(camera, MujocoCamera)
--> 496         else MujocoCamera.build(**camera)
    497     )
    499     _ = ET.SubElement(worldbody_element, "camera", mj_camera.asdict())
    501 # ------------------------------------------------
    502 # Add a light following the  CoM of the first link
    503 # ------------------------------------------------

TypeError: jaxsim.mujoco.loaders.MujocoCamera.build() argument after ** must be a mapping, not NoneType

---

📚 Documentation preview 📚: https://jaxsim--216.org.readthedocs.build//216/

[diegoferigo]

This PR is implementing a workaround when no camera is specified, but I think that the root problem is elsewhere. When I updated the cameras argument, I just realized that I didn't align also the UrdfToMjcf.convert method. Can you please update this PR such that in that method cameras=() instead of =None by default?

[flferretti]

You're right! Fixed with rebase

[diegoferigo]

Looks good! Thanks for the fix.