[backend/frontend] Invalidate empty CSV mappers (#4644)

[labo-flg]

Proposed changes

• new error case for the CSV mapper validator: having no representations. 1) This is useless and should not be selectable for import, and 2) this will throw error on using it during an import
• fix the CSV mapper create/edit form so that the TEST button is properly disabled when initial values are invalid

Related issues

• Improve error messaging on ImportCSV #4944

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

Further comments

I added some unit tests for the validator, but they do not cover all the cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4a90cb0) 65.59% compared to head (7c293ef) 65.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5272      +/-   ##
==========================================
+ Coverage   65.59%   65.61%   +0.02%     
==========================================
  Files         472      472              
  Lines       59160    59165       +5     
  Branches     4327     4336       +9     
==========================================
+ Hits        38808    38824      +16     
+ Misses      20352    20341      -11     

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