iox-#932 Handle nullptr callbacks in Listener

[dkroenke]

Pre-Review Checklist for the PR Author

1. Code follows the coding style of CONTRIBUTING.md
2. Tests follow the best practice for testing
3. Changelog updated in the unreleased section including API breaking changes
4. Branch follows the naming format (iox-#123-this-is-a-branch)
5. Commits messages are according to this guideline
   • Commit messages have the issue ID (iox-#123 commit text)
   • Commit messages are signed (git commit -s)
   • Commit author matches Eclipse Contributor Agreement (and ECA is signed)
6. Update the PR title
   • Follow the same conventions as for commit messages
   • Link to the relevant issue
7. Relevant issues are linked
8. Add sensible notes for the reviewer
9. All checks have passed (except task-list-completed)
10. Assign PR to reviewer

Notes for Reviewer

Implementation according to #933 (review)

Checklist for the PR Reviewer

• Commits are properly organized and messages are according to the guideline
• Code according to our coding style and naming conventions
• Unit tests have been written for new behavior
• Public API changes are documented via doxygen
• Copyright owner are updated in the changed files
• PR title describes the changes

Post-review Checklist for the PR Author

1. All open points are addressed and tracked via issues

References

• Closes Handle nullptr callbacks correctly in WaitSet and Listener #932

[codecov]

Codecov Report

Merging #937 (7801ec0) into master (0cd82bb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #937      +/-   ##
==========================================
+ Coverage   77.60%   77.62%   +0.01%     
==========================================
  Files         336      336              
  Lines       12309    12315       +6     
  Branches     1831     1833       +2     
==========================================
+ Hits         9553     9559       +6     
  Misses       2130     2130              
  Partials      626      626              

Flag	Coverage Δ
unittests	76.52% <81.81%> (+0.01%)	⬆️
unittests_timing	30.23% <45.45%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
iceoryx_binding_c/source/c_listener.cpp	78.57% <100.00%> (ø)
...ceoryx_binding_c/source/cpp2c_enum_translation.cpp	100.00% <100.00%> (ø)
...sh/include/iceoryx_posh/internal/popo/listener.inl	100.00% <100.00%> (ø)