Camera: fix default trigger topic

[scpeters]

🦟 Bug fix

Fixes default trigger topic when unset

Summary

In gazebosim/gz-sensors#215 (comment) I noticed that if the trigger topic name is not explicitly set in SDFormat that ign-sensors uses __default__/trigger as the default trigger topic name. I believe this is due to the way the DOM object is loaded, so I have adjusted it in 361fa6a to only read the value of the trigger_topic element if that element exists. Thus it will return an empty string "" if no trigger topic is specified, leaving the default behavior up to the downstream library (ign-sensors in the case I noticed).

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.

[codecov-commenter]

Codecov Report

Merging #1006 (fabc21b) into sdf12 (6424ffa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            sdf12    #1006   +/-   ##
=======================================
  Coverage   65.38%   65.38%           
=======================================
  Files           2        2           
  Lines          26       26           
=======================================
  Hits           17       17           
  Misses          9        9           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6424ffa...fabc21b. Read the comment docs.

[codecov]

Codecov Report

Merging #1006 (bc834eb) into sdf12 (a50ba52) will not change coverage.
The diff coverage is n/a.

❗ Current head bc834eb differs from pull request most recent head 2678029. Consider uploading reports for the commit 2678029 to get more accurate results

@@           Coverage Diff           @@
##            sdf12    #1006   +/-   ##
=======================================
  Coverage   65.38%   65.38%           
=======================================
  Files           2        2           
  Lines          26       26           
=======================================
  Hits           17       17           
  Misses          9        9           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a50ba52...2678029. Read the comment docs.