[ros2controlcli] fix list_controllers when no controllers are loaded (backport #1721)

[mergify]

Fixes: #1715

---

This is an automatic backport of pull request #1721 done by Mergify.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.30%. Comparing base (9eb64ad) to head (4558197).
Report is 2 commits behind head on humble.

Additional details and impacted files

@@           Coverage Diff           @@
##           humble    #1722   +/-   ##
=======================================
  Coverage   62.30%   62.30%           
=======================================
  Files         105      105           
  Lines       11314    11314           
  Branches     7925     7928    +3     
=======================================
  Hits         7049     7049           
  Misses        806      806           
  Partials     3459     3459           

Flag	Coverage Δ
unittests	62.30% <ø> (ø)

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