[Bug Report] Raycaster calling visualization with empty inputs

[DreaverZhao]

Describe the bug

If all rays of the raycaster sensors hit nothing within the max_distance, and we enable the debug visualization of them, we'll get ValueErrors printed in the terminal. It won't affect normal training though.

Steps to reproduce

Set up a raycaster (or its variants) with a given max_distance. For instance, I use the following setup (what really matters here is setting a max_distance and setting ray_alignment to 'base', other cfgs are not important).

height_sensor: MultiMeshRayCasterCfg = MultiMeshRayCasterCfg(
      prim_path="/World/envs/env_.*/Robot/body",
      update_period=0.02,
      offset=MultiMeshRayCasterCfg.OffsetCfg(pos=(0.05, 0.0, 0.0)),
      pattern_cfg=patterns.GridPatternCfg(resolution=0.02, size=[0.0, 0.0]),
      max_distance=4.0,
      debug_vis=True,
      ray_alignment='base',
      mesh_prim_paths=["/World/ground"]
)

Spawn an agent upside down, so that the sensor points towards the sky and the ray hits nothing. Then we'll see printed errors in the terminal similar to:

Traceback (most recent call last):
  File "/home/dreaver/IsaacLab/source/isaaclab/isaaclab/sensors/sensor_base.py", line 159, in <lambda>
    lambda event, obj=weakref.proxy(self): obj._debug_vis_callback(event)
                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dreaver/IsaacLab/source/isaaclab/isaaclab/sensors/ray_caster/ray_caster.py", line 329, in _debug_vis_callback
    self.ray_visualizer.visualize(viz_points)
  File "/home/dreaver/IsaacLab/source/isaaclab/isaaclab/markers/visualization_markers.py", line 330, in visualize
    raise ValueError("Number of markers cannot be zero! Hint: The function was called with no inputs?")
ValueError: Number of markers cannot be zero! Hint: The function was called with no inputs?

It won't affect the training or testing process. Yet it's a little bit annoying.

System Info

• Commit: [764aeb4]
• Isaac Sim Version: [5.1.0]
• OS: [Ubuntu 22.04]
• GPU: [RTX 4060Ti 16G]
• CUDA: [12.9]
• GPU Driver: [580.126.09]

Checklist

• I have checked that there is no similar issue in the repo (required)
• I have checked that the issue is not in running Isaac Sim itself and is related to the repo