Fix crash in OpticalTactilePlugin by checking for valid visualize pointer (backport #2674)

[mergify]

🦟 Bug fix

Fixes #2543

Summary

See steps in #2543 to reproduce the crash.

Calling the service to disable the plugin (when it has never been enabled before) will cause a crash because the optical tactile visualization marker pointer is null (not created yet). This PR adds a check to make sure the optical tactile visualization marker pointer is not null before attempting to remove the visualization markers.

Checklist

• 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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

---

This is an automatic backport of pull request #2674 done by [Mergify](https://mergify.com).