Fix adding parameter definition with parameter_type_list set to None

[soininen]

This fixes a Traceback when parameter definition was added with parameter_type_list set to None.

No associated issue.

Checklist before merging

• Documentation (also in Toolbox repo) 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 86.15%. Comparing base (82d214b) to head (7a030cd).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #619   +/-   ##
=======================================
  Coverage   86.15%   86.15%           
=======================================
  Files          83       83           
  Lines       11475    11475           
  Branches     1636     1636           
=======================================
  Hits         9886     9886           
  Misses       1238     1238           
  Partials      351      351           

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