Fix a bug that prevented the creation of new Data transformer specifications

[soininen]

No associated issue

Checklist before merging

• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 60.71%. Comparing base (569fa28) to head (96c0c2e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #190 +/- ## =========================================== + Coverage 60.66% 60.71% +0.05% =========================================== Files 193 193 Lines 17687 17688 +1 Branches 2875 2875 =========================================== + Hits 10729 10739 +10 + Misses 6375 6366 -9 Partials 583 583 ```

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