Fix swapped MJX ray empty-return types

[sylvesterkaczmarek]

Summary

When the ray filter excludes every geom, mjx.ray returns an integer in the distance slot and a float in the geom-id slot, contrary to its documented (distance, geom id) contract and its normal return path.

This change fixes the sentinel order, aligns the docstring, and adds a regression test asserting that the early-return dtypes match the normal path.

Context

Closed, unmerged PR #3326 identified the same bug. Current main still contains it and no active replacement was found.

Validation

• Based on current main
• Final diff changes only ray.py and ray_test.py
• Focused runtime validation was not run in this environment

[sylvesterkaczmarek]

@btaba thanks for taking the time to review and approve this. Much appreciated!