[backend] add needed execution message not blank constraint

[guillaumejparis]

Proposed changes

Related issues

• Closes Execution callback route authorize no message #2308

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality
• For bug fix -> I implemented a test that covers the bug

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...
…ith what's actually available

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release/current@a7343f0). Learn more about missing BASE report.
Report is 3 commits behind head on release/current.

Additional details and impacted files

@@                Coverage Diff                 @@
##             release/current    #2309   +/-   ##
==================================================
  Coverage                   ?   36.31%           
  Complexity                 ?     1658           
==================================================
  Files                      ?      589           
  Lines                      ?    18059           
  Branches                   ?     1176           
==================================================
  Hits                       ?     6558           
  Misses                     ?    11175           
  Partials                   ?      326           

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