Fix Traceback when copying empty rows from Parameter definition and value tables

[soininen]

Copying would cause a Traceback if default_value or value column was empty.

No associated issue.

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.82%. Comparing base (0f5c569) to head (9d875e1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3199   +/-   ##
=======================================
  Coverage   68.82%   68.82%           
=======================================
  Files         198      198           
  Lines       38336    38338    +2     
  Branches     4595     4596    +1     
=======================================
+ Hits        26386    26388    +2     
  Misses      10642    10642           
  Partials     1308     1308           

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