Fix issue in TopicViewer

[caguero]

🦟 Bug fix

See #430

Summary

This patch fixes a crash running the TopicViewer plugin. The code was assuming that all topics reported after calling TopicList() have publishers. This is no longer the case as we can also report subscribers.

See #430 to reproduce the issue/fix with the Gazebo Topic Viewer plugin.

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]

Codecov Report

Merging #566 (6fdeb04) into main (974957f) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 6fdeb04 differs from pull request most recent head 95c5b1f. Consider uploading reports for the commit 95c5b1f to get more accurate results

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
- Coverage   68.05%   68.03%   -0.03%     
==========================================
  Files          38       38              
  Lines        5394     5396       +2     
==========================================
  Hits         3671     3671              
- Misses       1723     1725       +2     

Files Changed	Coverage Δ
src/plugins/topic_viewer/TopicViewer.cc	80.64% <0.00%> (-1.06%)	⬇️