Add an error msg if empty message is received (backport #1424)

[mergify]

There existed one branch where a goal is rejected but no error message is given.

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This is an automatic backport of pull request #1424 done by Mergify.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.71%. Comparing base (7a2ad46) to head (29a41fa).
Report is 3 commits behind head on humble.

Files with missing lines	Patch %	Lines
...ory_controller/src/joint_trajectory_controller.cpp	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           humble    #1428   +/-   ##
=======================================
  Coverage   66.70%   66.71%           
=======================================
  Files         113      113           
  Lines       13216    13219    +3     
  Branches     8152     8153    +1     
=======================================
+ Hits         8816     8819    +3     
  Misses       1442     1442           
  Partials     2958     2958           

Flag	Coverage Δ
unittests	66.71% <83.33%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	80.84% <83.33%> (+0.07%)	⬆️