Fix detection of joint connecting world to base for fixed-base models

[diegoferigo]

Related to the changes introduced by #30

[flferretti]

When loading the model from urdf I get:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[6], line 1
----> 1 model = simulator.insert_model_from_description(
      2     model_description=model_urdf_path
      3 ).mutable(validate=True)

File [/conda/lib/python3.10/site-packages/jaxsim/simulation/simulator.py:242](https://vscode-remote+attached-002dcontainer-002b7b22636f6e7461696e65724e616d65223a222f62656175746966756c5f676f6c69636b227d.vscode-resource.vscode-cdn.net/conda/lib/python3.10/site-packages/jaxsim/simulation/simulator.py:242), in JaxSim.insert_model_from_description(self, model_description, model_name, considered_joints)
    228 """
    229 Insert a model from a model description.
    230 
   (...)
    238     The newly inserted model.
    239 """
    241 # Build the model from the given model description
--> 242 model = jaxsim.high_level.model.Model.build_from_model_description(
    243     model_description=model_description,
    244     model_name=model_name,
    245     vel_repr=self.velocity_representation,
    246     considered_joints=considered_joints,
    247 )
    249 # Make sure the model is not already part of the simulation
    250 if model.name() in self.model_names():

File [/conda/lib/python3.10/site-packages/jaxsim/high_level/model.py:153](https://vscode-remote+attached-002dcontainer-002b7b22636f6e7461696e65724e616d65223a222f62656175746966756c5f676f6c69636b227d.vscode-resource.vscode-cdn.net/conda/lib/python3.10/site-packages/jaxsim/high_level/model.py:153), in Model.build_from_model_description(model_description, model_name, vel_repr, gravity, is_urdf, considered_joints)
...
--> 186     raise ValueError("Pose of fixed joint connecting to 'world' link not valid")
    188 if j.pose.relative_to != j.parent:
    189     msg = "Pose of joint '{}' is not expressed wrt its parent link '{}'"

ValueError: Pose of fixed joint connecting to 'world' link not valid

[diegoferigo]

I update the PR. There was another occurrence to update.