Fix crash when running with --list-listeners and no registered listeners

[loximann]

Description

Running with --listeners without registered listeners caused Catch2 programs to crash.

$ ./a.out --list-listeners
zsh: segmentation fault (core dumped)  ./a.out --list-listeners

The reason was due to dereferencing an iterator pointing past the end of a vector when the vector is empty.

[codecov]

Codecov Report

Merging #2442 (7ae9ffc) into devel (cca5923) will decrease coverage by 0.01%.
The diff coverage is 66.67%.

❗ Current head 7ae9ffc differs from pull request most recent head 0057eb1. Consider uploading reports for the commit 0057eb1 to get more accurate results

@@            Coverage Diff             @@
##            devel    #2442      +/-   ##
==========================================
- Coverage   91.43%   91.42%   -0.01%     
==========================================
  Files         159      159              
  Lines        7501     7503       +2     
==========================================
+ Hits         6858     6859       +1     
- Misses        643      644       +1     

[loximann]

Should I add a test case to cover listing when no listeners are registered?

[horenmar]

Should I add a test case to cover listing when no listeners are registered?

Yes.

[loximann]

I modified the fix so that defaultListListeners is single return (which in general I think makes things cleaner). With that, no test cases are needed for full coverage; however I added a test case to avoid regressions.

[horenmar]

Please keep the early exit.

And coverage isn't really important, the regression check is.