Fix elevator system crash when empty laser scan msg is received.

[iche033]

🦟 Bug fix

Fixes https://github.com/gazebosim/gz-sim/issues/2531

Summary

See https://github.com/gazebosim/gz-sim/issues/2531 for instructions on reproducing the crash.

Added a check for non-empty laser scan ranges before accessing the elements.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
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