Loading non-fixed joint without inertial child throws "MissingComponentException"

[joaomacp]

Describe the bug
When loading a Revolute or Prismatic joint where the child link is missing <inertial> tags, a MissingComponentException exception is thrown.

To Reproduce
Steps to reproduce the behavior:

1. Load the example URDF below using URDF-Importer
2. The program crashes and a MissingComponentException is thrown: There is no 'ArticulationBody' attached to the "linkB" game object, but a script is trying to access it.
3. Uncommenting the <inertial> block loads the URDF correctly.

Console logs / stack traces

UnityEngine.MissingComponentException: There is no 'ArticulationBody' attached to the "linkB" game object, but a script is trying to access it.
You probably need to add a ArticulationBody to the game object "linkB". Or your script needs to check if the component is attached before using it.
  at (wrapper managed-to-native) UnityEngine.ArticulationBody.set_jointType(UnityEngine.ArticulationBody,UnityEngine.ArticulationJointType)
  at Unity.Robotics.UrdfImporter.UrdfJointRevolute.Create (UnityEngine.GameObject linkObject) [0x00012] in Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/UrdfComponents/UrdfJoints/UrdfJointRevolute.cs:28 
  at Unity.Robotics.UrdfImporter.UrdfJoint.AddCorrectJointType (UnityEngine.GameObject linkObject, Unity.Robotics.UrdfImporter.UrdfJoint+JointTypes jointType) [0x00034] in /Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/UrdfComponents/UrdfJoints/UrdfJoint.cs:113 
  at Unity.Robotics.UrdfImporter.UrdfJoint.Create (UnityEngine.GameObject linkObject, Unity.Robotics.UrdfImporter.UrdfJoint+JointTypes jointType, Unity.Robotics.UrdfImporter.Joint joint) [0x00000] in /Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/UrdfComponents/UrdfJoints/UrdfJoint.cs:89 
  at Unity.Robotics.UrdfImporter.UrdfLinkExtensions.ImportLinkData (Unity.Robotics.UrdfImporter.UrdfLink urdfLink, Unity.Robotics.UrdfImporter.Link link, Unity.Robotics.UrdfImporter.Joint joint) [0x0007a] in /Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/Extensions/UrdfLinkExtensions.cs:62 
  at Unity.Robotics.UrdfImporter.UrdfLinkExtensions.Create (UnityEngine.Transform parent, Unity.Robotics.UrdfImporter.Link link, Unity.Robotics.UrdfImporter.Joint joint) [0x00051] in Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/Extensions/UrdfLinkExtensions.cs:31 
  at Unity.Robotics.UrdfImporter.UrdfRobotExtensions.ProcessJointStack (Unity.Robotics.UrdfImporter.UrdfRobotExtensions+ImportPipelineData im) [0x00056] in Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/Extensions/UrdfRobotExtensions.cs:132 
  at Unity.Robotics.UrdfImporter.UrdfRobotExtensions.CreateRuntime (System.String filename, Unity.Robotics.UrdfImporter.ImportSettings settings) [0x00015] in Library/PackageCache/com.unity.robotics.urdf-importer@9956ce5368/Runtime/Extensions/UrdfRobotExtensions.cs:215

Expected behavior
IMO the program shouldn't crash. The joint could be ignored and not added to the structure, and thus not
controllable, since it misses <inertial> info for correct simulation. A warning about this should be logged.

The reason to not crash is that there are URDFs with joints that don't
need to be controlled directly: some parallel grippers contain joints
whose purpose is to be mimicked by other, physical joints. The
controlling joint doesn't need <inertial> info, only the mimic one.

This behaviour is similar to Gazebo, which also ignores links
without inertial info.

Environment:

• Unity Version: [e.g. Unity 2021.3.24f1]
• Unity machine OS + version: [e.g. Ubuntu 22.04]
• Branch or version: [e.g. URDF-Importer v0.5.2]

Example URDF

<robot name = "example">
        <link name = "base_link">
		<inertial>
			<origin xyz = "0 0 0" />
			<mass value = "0.5" />
			<inertia ixx = "0.5" iyy = "0.5" izz = "0.5" ixy = "0" ixz = "0" iyz = "0" />
		</inertial>
		<visual>
			<origin xyz = "0 0 0" />
			<geometry>
				<box size = "0.5 0.5 0.1" />
			</geometry>
			<material name = "gray A">
				<color rgba = "0.1 0.1 0.1 1" />
			</material>
		</visual>
	</link>
	<link name = "linkA">
		<inertial>
			<origin xyz = "0 0 0" />
			<mass value = "0.5" />
			<inertia ixx = "0.5" iyy = "0.5" izz = "0.5" ixy = "0" ixz = "0" iyz = "0" />
		</inertial>
		<visual>
			<origin xyz = "0 0 0" />
			<geometry>
				<box size = "0.5 0.5 0.1" />
			</geometry>
			<material name = "gray A">
				<color rgba = "0.1 0.1 0.1 1" />
			</material>
		</visual>
	</link>
	<link name = "linkB">
		<!-- Uncomment to add inertial info: this fixes the URDF-Importer exception -->
		<!--
		<inertial>
			<origin xyz = "0 0 -0.5" />
			<mass value = "0.5" />
			<inertia ixx = "0.5" iyy = "0.5" izz = "0.5" ixy = "0" ixz = "0" iyz = "0" />
		</inertial>
		-->
		<visual>
			<origin xyz = "0 0 -0.5" />
			<geometry>
				<cylinder radius = "0.05" length = "1"  />
			</geometry>
			<material name = "gray B">
				<color rgba = "0.3 0.3 0.3 1" />
			</material>
		</visual>
	</link>
       <joint name = "base_joint" type = "fixed">
		<parent link = "base_link" />
		<child link = "linkA" />
		<origin xyz = "0 0 -0.05" />
	</joint>
	<joint name = "joint AB" type = "revolute">
		<parent link = "linkA" />
		<child link = "linkB" />
		<origin xyz = "0 0 -0.05" />
		<axis xyz = "0 1 0" />
	</joint>
</robot>